diff --git a/.gitignore b/.gitignore
index 6490ecf..672fb67 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 /clonetool
+/dist/
diff --git a/README.md b/README.md
index fd7da02..824b68b 100644
--- a/README.md
+++ b/README.md
@@ -16,12 +16,20 @@ system `ssh` client for remote endpoints.
   A block is only re-read once: it's hashed from the buffer it was read
   into, and that same buffer is what gets sent on if it differs — never
   read twice.
+- **Source and destination are scanned in parallel.** The destination side
+  streams the hash of each block *as it computes it*, in block order; the
+  source side consumes that stream and immediately reads, compares and (if
+  it differs) sends that block. So the destination scan, the source scan
+  and the transfer of changed blocks all overlap — there is no "scan the
+  whole destination, then start" phase. The status line shows the transfer
+  progress with a trailing `scan H/M` until the destination fingerprint is
+  complete.
 - **No state is kept between runs.** Every sync re-reads and re-hashes the
   destination's *current* content and compares the source against that, so
   a re-run only moves the blocks that actually differ and nothing needs to
-  be trusted from a previous run. (The destination side does the
-  destination hashing; in push mode it streams that table to the source,
-  in pull mode the destination agent hashes locally.)
+  be trusted from a previous run. (The destination side always does the
+  destination hashing — the remote `sink` in push mode, the local
+  destination agent in pull mode.)
 - Bulk data goes **directly between source and destination**, not through
   the manager. Each run tries:
   1. **push** — the source agent connects straight to the destination
@@ -66,10 +74,54 @@ system `ssh` client for remote endpoints.
 CGO_ENABLED=0 go build -o clonetool .
 ```
 
+Cross-compile for another OS/arch by setting `GOOS`/`GOARCH` (no cgo, so
+these all work from any host):
+
+```
+CGO_ENABLED=0 GOOS=linux   GOARCH=arm64 go build -o clonetool        .
+CGO_ENABLED=0 GOOS=windows GOARCH=amd64 go build -o clonetool.exe    .
+CGO_ENABLED=0 GOOS=darwin  GOARCH=arm64 go build -o clonetool-darwin .
+```
+
+`./build.sh` writes all three of the above (plus linux/amd64) into `dist/`.
+
 Copy the resulting binary to the manager host. Source and destination
 hosts get it automatically (see self-deploy above), or place it yourself
 and point `--remote-bin` at it. The binary is architecture-specific —
-cross-compile (`GOOS`/`GOARCH`) if your hosts differ.
+cross-compile if your hosts differ. Self-deploy streams a POSIX shell
+script over SSH, so a **Windows** host can be the manager or a local
+endpoint but cannot be an automatic deploy target — put `clonetool.exe`
+on it yourself and point `--remote-bin` at it.
+
+## Windows
+
+clonetool runs on Windows and can clone **physical drives and volumes**,
+not just files:
+
+```
+# Whole disk to an image file
+clonetool sync --source \\.\PhysicalDrive2 --dest D:\backup\disk2.img
+
+# Image file back onto a disk (must not be larger than the disk)
+clonetool sync --source D:\backup\disk2.img --dest \\.\PhysicalDrive2
+
+# A single volume
+clonetool sync --source \\.\E: --dest \\.\F:
+```
+
+- Raw-disk paths are `\\.\PhysicalDrive<n>` (whole disk) or `\\.\<X>:` (a
+  volume). `\\?\` also works. Forward slashes are accepted.
+- **Run from an elevated (Administrator) console** to open a raw disk.
+  There is no `sudo` fallback on Windows; `--sudo` is ignored. A
+  permission error tells you to elevate.
+- A raw-disk **destination** should have no mounted filesystem in use
+  (take the disk offline in Disk Management, or target a volume that
+  nothing else has open) — Windows blocks writes to a disk region owned by
+  a mounted volume. Reading a live disk as the **source** is fine.
+- Raw-disk I/O must be sector-aligned. `--block-size` must be a multiple
+  of the drive's sector size (512 or 4096); the default 4M is. clonetool
+  handles the final partial block itself.
+- Device size is read with `IOCTL_DISK_GET_LENGTH_INFO`.
 
 ## Usage
 
@@ -111,16 +163,19 @@ the self-deploy check).
 
 ## Caveats
 
-- Block-device size detection (`BLKGETSIZE64`) is Linux-only.
+- Block-device size detection is implemented on Linux (`BLKGETSIZE64`) and
+  Windows (`IOCTL_DISK_GET_LENGTH_INFO`). On other systems only regular
+  files can be synced.
 - If a destination path doesn't exist yet, it's created as a regular
   file — clonetool won't create device nodes, so double-check device
   paths for typos before running.
 - SSH host keys are accepted on first connect (`StrictHostKeyChecking=accept-new`)
   and rejected if they later change, same as normal SSH behavior.
-- Because every run re-hashes the whole destination, a re-sync costs a
-  full read of both sides even when little changed — the win is in the
-  bytes transferred, not the bytes read. That read shows on the status
-  line as a `scanning destination` phase before `syncing` begins.
+- Because every run re-hashes both sides in full, a re-sync costs a full
+  read of source and destination even when little changed — the win is in
+  the bytes transferred, not the bytes read. The two reads run in
+  parallel; the status line shows a trailing `scan H/M` until the
+  destination fingerprint catches up.
 - `agent` is an internal subcommand spawned automatically by `sync`; it's
   not meant to be run by hand, though it will work standalone for
   debugging.
diff --git a/agent.go b/agent.go
index 668bb2a..5f4d451 100644
--- a/agent.go
+++ b/agent.go
@@ -75,29 +75,6 @@ func runControlAgent() error {
 	}
 }
 
-// readHashFrame reads frames off fr until the hash table arrives, relaying
-// any "scan" progress frames the sink sends while it hashes the destination
-// onward to the manager via out.
-func readHashFrame(fr *FrameReader, out *FrameWriter) ([][32]byte, error) {
-	for {
-		typ, payload, err := fr.ReadFrame()
-		if err != nil {
-			return nil, fmt.Errorf("read hash table: %w", err)
-		}
-		switch typ {
-		case frameHashTable:
-			return unflattenHashes(payload)
-		case frameCtrlJSON:
-			var m CtrlMsg
-			if json.Unmarshal(payload, &m) == nil && m.Type == msgProgress {
-				_ = out.WriteJSON(m)
-			}
-		default:
-			return nil, fmt.Errorf("expected hash table frame, got type %d", typ)
-		}
-	}
-}
-
 func handleStat(out *FrameWriter, m CtrlMsg) {
 	info, err := statPath(m.Path)
 	if err != nil {
@@ -183,16 +160,9 @@ func runPushDriver(req CtrlMsg, out *FrameWriter) {
 	}
 
 	// Handshake succeeded: we're committed to push for this run. The sink
-	// now sends the current per-block hashes of the destination it just
-	// read; the source loop compares against those to decide what to send.
-	hashes, err := readHashFrame(fr, out)
-	if err != nil {
-		_ = cmd.Process.Kill()
-		_ = cmd.Wait()
-		_ = out.WriteJSON(CtrlMsg{Type: msgError, Message: fmt.Sprintf("%v (remote stderr: %s)", err, stderrBuf.String())})
-		return
-	}
-
+	// streams the destination's current per-block hashes as it scans; the
+	// source loop consumes them in order and reads/compares its own blocks
+	// as they arrive, so the two scans and the transfer all overlap.
 	srcFile, err := os.Open(req.Path)
 	if err != nil {
 		_ = cmd.Process.Kill()
@@ -202,7 +172,7 @@ func runPushDriver(req CtrlMsg, out *FrameWriter) {
 	}
 	defer srcFile.Close()
 
-	if fatalErr := pumpPush(req, hashes, srcFile, fw, fr, out); fatalErr != nil {
+	if fatalErr := pumpPush(req, alignmentFor(req.Path), srcFile, fw, fr, out); fatalErr != nil {
 		_ = cmd.Process.Kill()
 		_ = cmd.Wait()
 		_ = out.WriteJSON(CtrlMsg{Type: msgError, Message: fatalErr.Error()})
@@ -246,10 +216,15 @@ type ackEvent struct {
 }
 
 // pumpPush runs the source-side read/hash/compare/send loop against fw
-// (the pipe to the remote sink) while concurrently draining ACK/ERR frames
-// from fr, so it can wait for every sent block's write to be confirmed
-// before declaring the push done.
-func pumpPush(req CtrlMsg, hashes [][32]byte, srcFile *os.File, fw *FrameWriter, fr *FrameReader, out *FrameWriter) error {
+// (the pipe to the remote sink) while a single reader goroutine over fr
+// demultiplexes the three things the sink sends back on that one stream:
+// the streamed destination block hashes (fed to the source loop as they
+// arrive, so its own scan overlaps the sink's), the ACK/ERR frames for
+// blocks it wrote, and any relayed scan-progress. It returns once every
+// sent block's write has been confirmed.
+func pumpPush(req CtrlMsg, align int64, srcFile *os.File, fw *FrameWriter, fr *FrameReader, out *FrameWriter) error {
+	blockCount := (req.Size + req.BlockSize - 1) / req.BlockSize
+
 	const maxInFlight = 32
 	sem := make(chan struct{}, maxInFlight)
 
@@ -265,15 +240,33 @@ func pumpPush(req CtrlMsg, hashes [][32]byte, srcFile *os.File, fw *FrameWriter,
 		lastProgress = time.Now()
 		_ = out.WriteJSON(CtrlMsg{
 			Type: msgProgress, Copied: atomic.LoadInt64(&copied), Skipped: atomic.LoadInt64(&skipped),
-			TotalBlocks: int64(len(hashes)),
+			TotalBlocks: blockCount,
 		})
 	}
 
+	// hashCh is sized to hold every block hash so the reader below never
+	// blocks handing hashes off (which, since ACKs share the same stream,
+	// would otherwise be able to deadlock against the in-flight-send limit).
+	// This is the same order of memory the old whole-table transfer used.
+	hashBuf := blockCount
+	if hashBuf < 1 {
+		hashBuf = 1
+	}
+	hashCh := make(chan blockHash, hashBuf)
+
 	ackEvents := make(chan ackEvent, 256)
 	go func() {
+		hashClosed := false
+		closeHash := func() {
+			if !hashClosed {
+				close(hashCh)
+				hashClosed = true
+			}
+		}
 		for {
 			typ, payload, err := fr.ReadFrame()
 			if err != nil {
+				closeHash()
 				if err == io.EOF {
 					ackEvents <- ackEvent{eof: true}
 				} else {
@@ -282,18 +275,36 @@ func pumpPush(req CtrlMsg, hashes [][32]byte, srcFile *os.File, fw *FrameWriter,
 				return
 			}
 			switch typ {
+			case frameBlockHash:
+				idx, h, derr := decodeBlockHashFrame(payload)
+				if derr != nil {
+					closeHash()
+					ackEvents <- ackEvent{err: derr}
+					return
+				}
+				hashCh <- blockHash{index: idx, hash: h}
+			case frameHashDone:
+				closeHash()
 			case frameAck:
-				idx, err := decodeIndexFrame(payload)
-				if err != nil {
-					ackEvents <- ackEvent{err: err}
+				idx, derr := decodeIndexFrame(payload)
+				if derr != nil {
+					closeHash()
+					ackEvents <- ackEvent{err: derr}
 					return
 				}
 				ackEvents <- ackEvent{index: idx}
 			case frameErr:
 				idx, msg, _ := decodeErrFrame(payload)
+				closeHash()
 				ackEvents <- ackEvent{err: fmt.Errorf("remote reported error at block %d: %s", idx, msg)}
 				return
+			case frameCtrlJSON:
+				var m CtrlMsg
+				if json.Unmarshal(payload, &m) == nil && m.Type == msgProgress {
+					_ = out.WriteJSON(m)
+				}
 			default:
+				closeHash()
 				ackEvents <- ackEvent{err: fmt.Errorf("unexpected frame type %d from sink", typ)}
 				return
 			}
@@ -303,7 +314,7 @@ func pumpPush(req CtrlMsg, hashes [][32]byte, srcFile *os.File, fw *FrameWriter,
 	sendErrCh := make(chan error, 1)
 	go func() {
 		sendErrCh <- runSourceLoop(sourceLoopParams{
-			File: srcFile, Size: req.Size, BlockSize: req.BlockSize, Hashes: hashes, Out: fw,
+			File: srcFile, Size: req.Size, BlockSize: req.BlockSize, Align: align, Hashes: hashCh, Out: fw,
 			OnSkip: func(uint64) { atomic.AddInt64(&skipped, 1); maybeProgress() },
 			OnSend: func(idx uint64, _ [32]byte) {
 				sem <- struct{}{}
@@ -373,7 +384,7 @@ func pumpPush(req CtrlMsg, hashes [][32]byte, srcFile *os.File, fw *FrameWriter,
 		return fmt.Errorf("sink closed the connection with %d block write confirmation(s) still outstanding", n)
 	}
 
-	_ = out.WriteJSON(CtrlMsg{Type: msgProgress, Copied: atomic.LoadInt64(&copied), Skipped: atomic.LoadInt64(&skipped), TotalBlocks: int64(len(hashes))})
+	_ = out.WriteJSON(CtrlMsg{Type: msgProgress, Copied: atomic.LoadInt64(&copied), Skipped: atomic.LoadInt64(&skipped), TotalBlocks: blockCount})
 	return nil
 }
 
@@ -421,9 +432,10 @@ func runPullDriver(req CtrlMsg, out *FrameWriter) {
 		return
 	}
 
-	// Handshake succeeded. Open the destination, fingerprint its current
-	// content block by block, and hand that table to the source stream so it
-	// only sends back what differs.
+	// Handshake succeeded. Open the destination and fingerprint its current
+	// content block by block, streaming each hash to the source stream the
+	// moment it is computed so it can start comparing straight away; write
+	// whatever it streams back into the same file concurrently.
 	dstFile, err := os.OpenFile(req.Path, os.O_RDWR, 0)
 	if err != nil {
 		_ = cmd.Process.Kill()
@@ -432,30 +444,40 @@ func runPullDriver(req CtrlMsg, out *FrameWriter) {
 		return
 	}
 	defer dstFile.Close()
+	align := alignmentFor(req.Path)
 
-	hashes, err := hashFileBlocks(dstFile, req.Size, req.BlockSize, scanProgressEmitter(out))
-	if err != nil {
-		_ = cmd.Process.Kill()
-		_ = cmd.Wait()
-		_ = out.WriteJSON(CtrlMsg{Type: msgError, Message: err.Error(), NeedPriv: isPermErr(err)})
-		return
-	}
-	if err := fw.WriteFrame(frameHashTable, flattenHashes(hashes)); err != nil {
-		_ = cmd.Process.Kill()
-		_ = cmd.Wait()
-		_ = out.WriteJSON(CtrlMsg{Type: msgError, Message: fmt.Sprintf("send hash table: %v", err)})
-		return
-	}
+	hashErrCh := make(chan error, 1)
+	go func() {
+		err := streamHashBlocks(dstFile, req.Size, req.BlockSize, align, func(bh blockHash) error {
+			return fw.WriteFrame(frameBlockHash, encodeBlockHashFrame(bh.index, bh.hash))
+		}, scanProgressEmitter(out))
+		if err == nil {
+			err = fw.WriteFrame(frameHashDone, nil)
+		}
+		if err != nil {
+			// Unblock the source stream (waiting for more hashes) so the
+			// dest loop below can unwind instead of hanging.
+			_ = stdin.Close()
+		}
+		hashErrCh <- err
+	}()
 
 	loopErr := runDestLoop(destLoopParams{
-		File: dstFile, BlockSize: req.BlockSize, In: fr,
+		File: dstFile, BlockSize: req.BlockSize, Align: align, In: fr,
 		OnCtrlMsg: func(m CtrlMsg) {
 			if m.Type == msgProgress {
 				_ = out.WriteJSON(m)
 			}
 		},
 	})
+	hashErr := <-hashErrCh
 
+	if hashErr != nil {
+		_ = cmd.Process.Kill()
+		_ = cmd.Wait()
+		_ = out.WriteJSON(CtrlMsg{Type: msgError, Message: fmt.Sprintf("scan destination: %v", hashErr), NeedPriv: isPermErr(hashErr)})
+		return
+	}
 	if loopErr != nil {
 		_ = cmd.Process.Kill()
 		_ = cmd.Wait()
@@ -484,6 +506,7 @@ func runSinkRole(path string, size, blockSize int64) error {
 		return err
 	}
 	defer f.Close()
+	align := alignmentFor(path)
 
 	// Answer the handshake immediately so the push driver's short readiness
 	// timeout isn't spent hashing a large destination.
@@ -491,23 +514,37 @@ func runSinkRole(path string, size, blockSize int64) error {
 		return err
 	}
 
-	hashes, err := hashFileBlocks(f, size, blockSize, scanProgressEmitter(out))
-	if err != nil {
-		fmt.Fprintf(os.Stderr, "sink: hash %s: %v\n", path, err)
-		return err
-	}
-	if err := out.WriteFrame(frameHashTable, flattenHashes(hashes)); err != nil {
-		return err
-	}
+	// Stream the destination's per-block hashes as they're computed while the
+	// dest loop below concurrently receives and writes changed blocks. A
+	// block is always hashed before a write for it can arrive (the source
+	// only sends after seeing that block's hash), so the two accesses to f
+	// never race on the same region.
+	hashErrCh := make(chan error, 1)
+	go func() {
+		err := streamHashBlocks(f, size, blockSize, align, func(bh blockHash) error {
+			return out.WriteFrame(frameBlockHash, encodeBlockHashFrame(bh.index, bh.hash))
+		}, scanProgressEmitter(out))
+		if err == nil {
+			err = out.WriteFrame(frameHashDone, nil)
+		} else {
+			fmt.Fprintf(os.Stderr, "sink: hash %s: %v\n", path, err)
+			_ = out.WriteFrame(frameErr, encodeErrFrame(0, err.Error()))
+		}
+		hashErrCh <- err
+	}()
 
-	return runDestLoop(destLoopParams{File: f, BlockSize: blockSize, In: in, AckOut: out})
+	loopErr := runDestLoop(destLoopParams{File: f, BlockSize: blockSize, Align: align, In: in, AckOut: out})
+	if hashErr := <-hashErrCh; hashErr != nil {
+		return hashErr
+	}
+	return loopErr
 }
 
 // ---------------------------------------------------------------------
 // source-stream role: one-shot process spawned (over ssh, in pull mode) on
-// the source host. Reads the hash table, then performs the same
-// read/hash/compare/send loop a local push driver would, writing straight
-// to its own stdout.
+// the source host. Consumes the streamed destination block hashes, then
+// performs the same read/hash/compare/send loop a local push driver would,
+// writing straight to its own stdout.
 // ---------------------------------------------------------------------
 
 func runSourceStreamRole(path string, size, blockSize int64) error {
@@ -520,22 +557,47 @@ func runSourceStreamRole(path string, size, blockSize int64) error {
 		return err
 	}
 	defer f.Close()
+	align := alignmentFor(path)
 
 	if err := out.WriteFrame(frameReady, nil); err != nil {
 		return err
 	}
 
-	typ, payload, err := in.ReadFrame()
-	if err != nil {
-		return fmt.Errorf("source-stream: read hash table: %w", err)
-	}
-	if typ != frameHashTable {
-		return fmt.Errorf("source-stream: expected hash table frame, got type %d", typ)
-	}
-	hashes, err := unflattenHashes(payload)
-	if err != nil {
-		return err
+	blockCount := (size + blockSize - 1) / blockSize
+	hashBuf := blockCount
+	if hashBuf < 1 {
+		hashBuf = 1
 	}
+	hashCh := make(chan blockHash, hashBuf)
+	readErrCh := make(chan error, 1)
+	go func() {
+		for {
+			typ, payload, err := in.ReadFrame()
+			if err != nil {
+				readErrCh <- fmt.Errorf("source-stream: read hash stream: %w", err)
+				close(hashCh)
+				return
+			}
+			switch typ {
+			case frameBlockHash:
+				idx, h, derr := decodeBlockHashFrame(payload)
+				if derr != nil {
+					readErrCh <- derr
+					close(hashCh)
+					return
+				}
+				hashCh <- blockHash{index: idx, hash: h}
+			case frameHashDone:
+				readErrCh <- nil
+				close(hashCh)
+				return
+			default:
+				readErrCh <- fmt.Errorf("source-stream: unexpected frame type %d", typ)
+				close(hashCh)
+				return
+			}
+		}
+	}()
 
 	var copied, skipped int64
 	var lastProgress time.Time
@@ -544,12 +606,16 @@ func runSourceStreamRole(path string, size, blockSize int64) error {
 			return
 		}
 		lastProgress = time.Now()
-		_ = out.WriteJSON(CtrlMsg{Type: msgProgress, Copied: copied, Skipped: skipped, TotalBlocks: int64(len(hashes))})
+		_ = out.WriteJSON(CtrlMsg{Type: msgProgress, Copied: copied, Skipped: skipped, TotalBlocks: blockCount})
 	}
 
-	return runSourceLoop(sourceLoopParams{
-		File: f, Size: size, BlockSize: blockSize, Hashes: hashes, Out: out,
+	loopErr := runSourceLoop(sourceLoopParams{
+		File: f, Size: size, BlockSize: blockSize, Align: align, Hashes: hashCh, Out: out,
 		OnSkip: func(uint64) { skipped++; maybeProgress() },
 		OnSend: func(uint64, [32]byte) { copied++; maybeProgress() },
 	})
+	if readErr := <-readErrCh; readErr != nil {
+		return readErr
+	}
+	return loopErr
 }
diff --git a/build.sh b/build.sh
new file mode 100755
index 0000000..8585b1d
--- /dev/null
+++ b/build.sh
@@ -0,0 +1,21 @@
+#!/bin/sh
+# Build static clonetool binaries for the common OS/arch targets into dist/.
+set -e
+cd "$(dirname "$0")"
+mkdir -p dist
+
+build() {
+	os=$1 arch=$2 out=$3
+	echo "==> $os/$arch -> dist/$out"
+	CGO_ENABLED=0 GOOS="$os" GOARCH="$arch" go build -trimpath -ldflags "-s -w" -o "dist/$out" .
+}
+
+build linux   amd64 clonetool-linux-amd64
+build linux   arm64 clonetool-linux-arm64
+build windows amd64 clonetool-windows-amd64.exe
+build windows arm64 clonetool-windows-arm64.exe
+build darwin  amd64 clonetool-darwin-amd64
+build darwin  arm64 clonetool-darwin-arm64
+
+echo "done:"
+ls -la dist/
diff --git a/control.go b/control.go
index 61d1dfa..30eede4 100644
--- a/control.go
+++ b/control.go
@@ -197,22 +197,3 @@ func (c *Controller) Close() error {
 	}
 	return nil
 }
-
-func flattenHashes(hashes [][32]byte) []byte {
-	buf := make([]byte, len(hashes)*32)
-	for i, h := range hashes {
-		copy(buf[i*32:], h[:])
-	}
-	return buf
-}
-
-func unflattenHashes(b []byte) ([][32]byte, error) {
-	if len(b)%32 != 0 {
-		return nil, fmt.Errorf("hash table: %d bytes is not a multiple of 32", len(b))
-	}
-	out := make([][32]byte, len(b)/32)
-	for i := range out {
-		copy(out[i][:], b[i*32:i*32+32])
-	}
-	return out, nil
-}
diff --git a/ctrlmsg.go b/ctrlmsg.go
index 0553082..6ed4ccb 100644
--- a/ctrlmsg.go
+++ b/ctrlmsg.go
@@ -39,8 +39,11 @@ type CtrlMsg struct {
 	Reason  string `json:"reason,omitempty"`
 	Message string `json:"message,omitempty"`
 
-	// progress
+	// progress. Scan and transfer now overlap, so a run emits both "scan"
+	// messages (Hashed/TotalBlocks) and transfer messages (Copied/Skipped)
+	// interleaved; the printer keeps the latest of each.
 	Phase       string `json:"phase,omitempty"` // "scan" while hashing the destination; empty during transfer
+	Hashed      int64  `json:"hashed,omitempty"`
 	Copied      int64  `json:"copied,omitempty"`
 	Skipped     int64  `json:"skipped,omitempty"`
 	TotalBlocks int64  `json:"totalBlocks,omitempty"`
diff --git a/device.go b/device.go
index 068cdd0..6a9a0c2 100644
--- a/device.go
+++ b/device.go
@@ -14,6 +14,19 @@ type PathInfo struct {
 }
 
 func statPath(path string) (PathInfo, error) {
+	// A raw disk handle (Windows \\.\PhysicalDrive0, \\.\C:) is not something
+	// os.Stat can describe, so ask the platform for its size directly.
+	if isDevicePath(path) {
+		size, err := blockDeviceSize(path)
+		if err != nil {
+			if os.IsNotExist(err) {
+				return PathInfo{Exists: false}, nil
+			}
+			return PathInfo{}, fmt.Errorf("stat device %s: %w", path, err)
+		}
+		return PathInfo{Exists: true, IsDevice: true, Size: size}, nil
+	}
+
 	fi, err := os.Stat(path)
 	if err != nil {
 		if os.IsNotExist(err) {
@@ -50,6 +63,9 @@ func prepareDest(path string, targetSize int64) error {
 		}
 		return nil
 	}
+	if isDevicePath(path) && !info.Exists {
+		return fmt.Errorf("destination device %s not found", path)
+	}
 	f, err := os.OpenFile(path, os.O_CREATE|os.O_WRONLY, 0644)
 	if err != nil {
 		return fmt.Errorf("open/create %s: %w", path, err)
diff --git a/device_other.go b/device_other.go
index ae13f7e..0370ed7 100644
--- a/device_other.go
+++ b/device_other.go
@@ -1,9 +1,9 @@
-//go:build !linux
+//go:build !linux && !windows
 
 package main
 
 import "fmt"
 
 func blockDeviceSize(path string) (int64, error) {
-	return 0, fmt.Errorf("block device size detection is only implemented on linux (got path %s)", path)
+	return 0, fmt.Errorf("block device size detection is only implemented on linux and windows (got path %s)", path)
 }
diff --git a/device_unix.go b/device_unix.go
new file mode 100644
index 0000000..11e9aa4
--- /dev/null
+++ b/device_unix.go
@@ -0,0 +1,18 @@
+//go:build !windows
+
+package main
+
+// isDevicePath reports whether a path must be treated as a raw device from
+// its string form alone. On unix, os.Stat's mode bits already identify
+// device nodes accurately (symlinks, non-standard locations and all), so
+// this is always false and statPath relies on those instead.
+func isDevicePath(string) bool { return false }
+
+// alignmentFor returns the offset/length alignment a path's handle requires
+// for positioned reads and writes. Unix block devices accept ordinary
+// buffered pread/pwrite at any alignment, so there is nothing to round to.
+func alignmentFor(string) int64 { return 1 }
+
+// canElevate reports whether a permission failure opening a device is worth
+// retrying under `sudo` (see --sudo). Always true on unix.
+func canElevate() bool { return true }
diff --git a/device_windows.go b/device_windows.go
new file mode 100644
index 0000000..abd4969
--- /dev/null
+++ b/device_windows.go
@@ -0,0 +1,99 @@
+//go:build windows
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"syscall"
+	"unsafe"
+)
+
+// isDevicePath recognises the Win32 device namespaces clonetool can sync
+// against by string form: \\.\PhysicalDrive0 (a whole disk) and \\.\C: (a
+// volume). Both the \\.\ and \\?\ prefixes, and their forward-slash
+// spellings, are accepted. os.Stat can't describe these paths, so statPath
+// keys off this instead of mode bits on Windows.
+func isDevicePath(path string) bool {
+	p := strings.ReplaceAll(path, "/", `\`)
+	return strings.HasPrefix(p, `\\.\`) || strings.HasPrefix(p, `\\?\`)
+}
+
+// canElevate is false on Windows: there is no `sudo` equivalent to re-exec
+// under, so a permission failure opening a raw disk is reported with a hint
+// to run from an elevated console instead of being retried.
+func canElevate() bool { return false }
+
+const (
+	ioctlDiskGetLengthInfo    = 0x0007405C // IOCTL_DISK_GET_LENGTH_INFO
+	ioctlDiskGetDriveGeometry = 0x00070000 // IOCTL_DISK_GET_DRIVE_GEOMETRY
+)
+
+// diskGeometry mirrors DISK_GEOMETRY (24 bytes; Cylinders is a LARGE_INTEGER).
+type diskGeometry struct {
+	Cylinders         int64
+	MediaType         uint32
+	TracksPerCylinder uint32
+	SectorsPerTrack   uint32
+	BytesPerSector    uint32
+}
+
+// openDeviceHandle opens path for a metadata ioctl only: zero access rights
+// (which need no privilege and don't require the volume to be unlocked) and
+// shared read/write so it doesn't disturb a mounted filesystem.
+func openDeviceHandle(path string) (syscall.Handle, error) {
+	p, err := syscall.UTF16PtrFromString(path)
+	if err != nil {
+		return syscall.InvalidHandle, err
+	}
+	return syscall.CreateFile(p, 0,
+		syscall.FILE_SHARE_READ|syscall.FILE_SHARE_WRITE, nil,
+		syscall.OPEN_EXISTING, 0, 0)
+}
+
+// blockDeviceSize returns the byte length of a physical drive or volume via
+// DeviceIoControl(IOCTL_DISK_GET_LENGTH_INFO).
+func blockDeviceSize(path string) (int64, error) {
+	h, err := openDeviceHandle(path)
+	if err != nil {
+		return 0, &os.PathError{Op: "open", Path: path, Err: err}
+	}
+	defer syscall.CloseHandle(h)
+
+	var length int64 // GET_LENGTH_INFORMATION is a single LARGE_INTEGER
+	var ret uint32
+	err = syscall.DeviceIoControl(h, ioctlDiskGetLengthInfo,
+		nil, 0,
+		(*byte)(unsafe.Pointer(&length)), uint32(unsafe.Sizeof(length)),
+		&ret, nil)
+	if err != nil {
+		return 0, fmt.Errorf("IOCTL_DISK_GET_LENGTH_INFO %s: %w", path, err)
+	}
+	return length, nil
+}
+
+// alignmentFor returns the sector size a raw disk handle's positioned reads
+// and writes must be aligned to; 1 for an ordinary file path. It falls back
+// to 512 if the geometry query fails.
+func alignmentFor(path string) int64 {
+	if !isDevicePath(path) {
+		return 1
+	}
+	h, err := openDeviceHandle(path)
+	if err != nil {
+		return 512
+	}
+	defer syscall.CloseHandle(h)
+
+	var g diskGeometry
+	var ret uint32
+	err = syscall.DeviceIoControl(h, ioctlDiskGetDriveGeometry,
+		nil, 0,
+		(*byte)(unsafe.Pointer(&g)), uint32(unsafe.Sizeof(g)),
+		&ret, nil)
+	if err != nil || g.BytesPerSector == 0 {
+		return 512
+	}
+	return int64(g.BytesPerSector)
+}
diff --git a/frame.go b/frame.go
index fcef468..7579d2c 100644
--- a/frame.go
+++ b/frame.go
@@ -22,12 +22,13 @@ type frameType byte
 
 const (
 	frameCtrlJSON  frameType = 0x01 // payload: JSON object (control channel)
-	frameHashTable frameType = 0x02 // payload: raw concatenated 32-byte hashes
+	frameBlockHash frameType = 0x02 // payload: 8-byte index + 32-byte hash (one destination block, streamed in index order)
 	frameData      frameType = 0x03 // payload: 8-byte index + 32-byte hash + block bytes
 	frameAck       frameType = 0x04 // payload: 8-byte index
 	frameErr       frameType = 0x05 // payload: 8-byte index + UTF-8 message
 	frameDone      frameType = 0x06 // payload: empty
 	frameReady     frameType = 0x07 // payload: empty
+	frameHashDone  frameType = 0x08 // payload: empty — end of the frameBlockHash stream
 )
 
 // FrameWriter serializes concurrent writers onto one underlying stream.
@@ -108,6 +109,22 @@ func decodeDataFrame(b []byte) (index uint64, hash [32]byte, payload []byte, err
 	return index, hash, payload, nil
 }
 
+func encodeBlockHashFrame(index uint64, hash [32]byte) []byte {
+	buf := make([]byte, 8+32)
+	binary.BigEndian.PutUint64(buf[0:8], index)
+	copy(buf[8:40], hash[:])
+	return buf
+}
+
+func decodeBlockHashFrame(b []byte) (index uint64, hash [32]byte, err error) {
+	if len(b) < 40 {
+		return 0, hash, fmt.Errorf("block-hash frame too short: %d bytes", len(b))
+	}
+	index = binary.BigEndian.Uint64(b[0:8])
+	copy(hash[:], b[8:40])
+	return index, hash, nil
+}
+
 func encodeIndexFrame(index uint64) []byte {
 	buf := make([]byte, 8)
 	binary.BigEndian.PutUint64(buf, index)
diff --git a/manager.go b/manager.go
index 28683e7..ac05113 100644
--- a/manager.go
+++ b/manager.go
@@ -144,7 +144,7 @@ func runSync(cfg SyncConfig) error {
 // restart under sudo. The returned bool reports whether the agent (and any
 // peer helper it later spawns for this side) is running elevated.
 func bringUpController(spec Spec, tag string, cfg *SyncConfig, remoteBin, probePath string) (*Controller, PathInfo, bool, error) {
-	sudo := cfg.Sudo == "always"
+	sudo := cfg.Sudo == "always" && canElevate()
 	c, err := startController(spec, tag, cfg, remoteBin, sudo)
 	if err != nil {
 		return nil, PathInfo{}, sudo, err
@@ -153,6 +153,11 @@ func bringUpController(spec Spec, tag string, cfg *SyncConfig, remoteBin, probeP
 	if err == nil {
 		return c, info, sudo, nil
 	}
+	if errors.Is(err, errNeedPriv) && !canElevate() {
+		c.Close()
+		return nil, PathInfo{}, sudo, fmt.Errorf(
+			"%w; on Windows, run clonetool from an elevated (Administrator) console to open a raw disk", err)
+	}
 	if errors.Is(err, errNeedPriv) && cfg.Sudo == "auto" && !sudo {
 		fmt.Fprintf(os.Stderr, "%s: permission denied on %s; retrying via sudo ...\n", tag, probePath)
 		c.Close()
@@ -229,9 +234,11 @@ func confirmShrink(dstSpec Spec, oldSize, newSize int64) bool {
 	return line == "y" || line == "yes"
 }
 
-// progressPrinter renders a single rewriting status line on stderr for both
-// phases of a run: "scan" while a side hashes the destination, then the
-// block transfer itself (with a rolling copy rate).
+// progressPrinter renders a single rewriting status line on stderr. Scan
+// and transfer now run concurrently, so it keeps the latest of each kind of
+// update (blocks hashed so far, blocks copied/skipped) and shows them on one
+// line, with a rolling copy rate; the "scan H/M" segment disappears once the
+// destination fingerprint is complete.
 type progressPrinter struct {
 	blockSize  int64
 	start      time.Time
@@ -239,6 +246,11 @@ type progressPrinter struct {
 	lastCopied int64
 	rate       float64 // copied blocks/sec, smoothed
 	active     bool
+
+	hashed  int64
+	copied  int64
+	skipped int64
+	total   int64
 }
 
 func newProgressPrinter(blockSize int64) *progressPrinter {
@@ -248,34 +260,41 @@ func newProgressPrinter(blockSize int64) *progressPrinter {
 
 func (p *progressPrinter) print(m CtrlMsg) {
 	p.active = true
+	if m.TotalBlocks > 0 {
+		p.total = m.TotalBlocks
+	}
 	if m.Phase == "scan" {
-		fmt.Fprintf(os.Stderr, "\r  scanning destination %s %s/%s blocks          ",
-			progressBar(m.Copied, m.TotalBlocks),
-			formatCount(m.Copied), formatCount(m.TotalBlocks))
-		return
+		if m.Hashed > p.hashed {
+			p.hashed = m.Hashed
+		}
+	} else {
+		p.copied, p.skipped = m.Copied, m.Skipped
+		now := time.Now()
+		if dt := now.Sub(p.lastT).Seconds(); dt >= 0.4 {
+			inst := float64(m.Copied-p.lastCopied) / dt
+			if p.rate == 0 {
+				p.rate = inst
+			} else {
+				p.rate = 0.6*p.rate + 0.4*inst
+			}
+			p.lastT, p.lastCopied = now, m.Copied
+		}
 	}
 
-	done := m.Copied + m.Skipped
-	total := m.TotalBlocks
+	done := p.copied + p.skipped
+	total := p.total
 	if total <= 0 {
 		total = done
 	}
-
-	now := time.Now()
-	if dt := now.Sub(p.lastT).Seconds(); dt >= 0.4 {
-		inst := float64(m.Copied-p.lastCopied) / dt
-		if p.rate == 0 {
-			p.rate = inst
-		} else {
-			p.rate = 0.6*p.rate + 0.4*inst
-		}
-		p.lastT, p.lastCopied = now, m.Copied
-	}
 	mib := p.rate * float64(p.blockSize) / (1024 * 1024)
 
-	fmt.Fprintf(os.Stderr, "\r  syncing %s %s/%s  copied=%s skipped=%s  %6.1f MiB/s          ",
+	scan := ""
+	if p.total > 0 && p.hashed < p.total {
+		scan = fmt.Sprintf("  scan %s/%s", formatCount(p.hashed), formatCount(p.total))
+	}
+	fmt.Fprintf(os.Stderr, "\r  syncing %s %s/%s  copied=%s skipped=%s%s  %6.1f MiB/s        ",
 		progressBar(done, total), formatCount(done), formatCount(total),
-		formatCount(m.Copied), formatCount(m.Skipped), mib)
+		formatCount(p.copied), formatCount(p.skipped), scan, mib)
 }
 
 // finish ends the current status line with a newline so following output
diff --git a/syncside.go b/syncside.go
index a898a3b..f3b2c9c 100644
--- a/syncside.go
+++ b/syncside.go
@@ -9,10 +9,13 @@ import (
 	"time"
 )
 
-// readBlockAt reads exactly min(len(buf), totalSize-offset) bytes at offset,
-// treating a fully-satisfied read as success even if the underlying
-// implementation also reports io.EOF for it.
-func readBlockAt(f *os.File, buf []byte, offset, totalSize int64) (int, error) {
+// readBlockAt reads exactly min(len(buf), totalSize-offset) logical bytes at
+// offset, treating a fully-satisfied read as success even if the underlying
+// implementation also reports io.EOF for it. When align > 1 (a raw disk
+// handle that only accepts sector-aligned I/O) the physical read is rounded
+// up to the next multiple of align, but the returned count is still the
+// logical size — the caller only ever looks at buf[:n].
+func readBlockAt(f *os.File, buf []byte, offset, totalSize, align int64) (int, error) {
 	remaining := totalSize - offset
 	if remaining <= 0 {
 		return 0, io.EOF
@@ -21,40 +24,94 @@ func readBlockAt(f *os.File, buf []byte, offset, totalSize int64) (int, error) {
 	if remaining < want {
 		want = remaining
 	}
-	n, err := f.ReadAt(buf[:want], offset)
-	if err != nil && !(err == io.EOF && int64(n) == want) {
+	readLen := want
+	if align > 1 && want%align != 0 {
+		readLen = roundUp(want, align)
+		if readLen > int64(len(buf)) {
+			readLen = int64(len(buf))
+		}
+	}
+	n, err := f.ReadAt(buf[:readLen], offset)
+	if err != nil && !(err == io.EOF && int64(n) >= want) {
 		return n, err
 	}
+	if int64(n) > want {
+		n = int(want)
+	}
 	return n, nil
 }
 
-// hashFileBlocks reads f in blockSize-byte blocks up to size and returns the
-// SHA-256 of each. This is how a destination fingerprints its *current*
-// content at the start of every sync — clonetool keeps no hash state of its
-// own between runs. onProgress, if non-nil, is called with (blocksHashed,
-// totalBlocks) before the first block and after each one.
-func hashFileBlocks(f *os.File, size, blockSize int64, onProgress func(done, total int64)) ([][32]byte, error) {
+// writeBlockAt writes block at offset. When align > 1 and the block is not a
+// whole number of sectors (only ever the final block of a device sync), the
+// enclosing aligned span is read first and the block overlaid onto it, so a
+// raw disk handle that rejects sub-sector writes still gets an aligned write
+// and the bytes past the sync size are preserved.
+func writeBlockAt(f *os.File, block []byte, offset, align int64) error {
+	if align <= 1 || int64(len(block))%align == 0 {
+		_, err := f.WriteAt(block, offset)
+		return err
+	}
+	padded := roundUp(int64(len(block)), align)
+	tmp := make([]byte, padded)
+	if _, err := f.ReadAt(tmp, offset); err != nil && err != io.EOF {
+		return fmt.Errorf("read-modify-write tail at %d: %w", offset, err)
+	}
+	copy(tmp, block)
+	_, err := f.WriteAt(tmp, offset)
+	return err
+}
+
+// checkBlockAlign rejects a block size that a raw disk handle's sector
+// alignment can't satisfy (every block offset is a multiple of the block
+// size, so the block size itself must be a whole number of sectors).
+func checkBlockAlign(blockSize, align int64) error {
+	if align > 1 && blockSize%align != 0 {
+		return fmt.Errorf("block size %d is not a multiple of the device's %d-byte sector size; pass --block-size divisible by %d",
+			blockSize, align, align)
+	}
+	return nil
+}
+
+// blockHash pairs a block index with the SHA-256 of the destination's
+// current content for that block. The destination side streams these in
+// ascending index order as it scans; the source side consumes them in the
+// same order, so the two scans overlap instead of running back to back.
+type blockHash struct {
+	index uint64
+	hash  [32]byte
+}
+
+// streamHashBlocks reads f in blockSize-byte blocks up to size, hashing each
+// one and handing it to onHash the moment it is computed (this is what lets
+// the peer start comparing before the whole side has been scanned). It keeps
+// no hash state of its own. onProgress, if non-nil, is called with
+// (blocksHashed, totalBlocks) before the first block and after each one.
+func streamHashBlocks(f *os.File, size, blockSize, align int64, onHash func(bh blockHash) error, onProgress func(done, total int64)) error {
+	if err := checkBlockAlign(blockSize, align); err != nil {
+		return err
+	}
 	blockCount := (size + blockSize - 1) / blockSize
-	out := make([][32]byte, blockCount)
 	buf := make([]byte, blockSize)
 	if onProgress != nil {
 		onProgress(0, blockCount)
 	}
 	for i := int64(0); i < blockCount; i++ {
-		n, err := readBlockAt(f, buf, i*blockSize, size)
+		n, err := readBlockAt(f, buf, i*blockSize, size, align)
 		if err != nil {
-			return nil, fmt.Errorf("hash %s at block %d: %w", f.Name(), i, err)
+			return fmt.Errorf("hash %s at block %d: %w", f.Name(), i, err)
+		}
+		if err := onHash(blockHash{index: uint64(i), hash: sha256.Sum256(buf[:n])}); err != nil {
+			return err
 		}
-		out[i] = sha256.Sum256(buf[:n])
 		if onProgress != nil {
 			onProgress(i+1, blockCount)
 		}
 	}
-	return out, nil
+	return nil
 }
 
-// scanProgressEmitter returns an onProgress callback for hashFileBlocks that
-// forwards "scan" phase progress onto out, throttled to ~3/second.
+// scanProgressEmitter returns an onProgress callback for streamHashBlocks
+// that forwards "scan" phase progress onto out, throttled to ~3/second.
 func scanProgressEmitter(out *FrameWriter) func(done, total int64) {
 	var last time.Time
 	return func(done, total int64) {
@@ -62,46 +119,55 @@ func scanProgressEmitter(out *FrameWriter) func(done, total int64) {
 			return
 		}
 		last = time.Now()
-		_ = out.WriteJSON(CtrlMsg{Type: msgProgress, Phase: "scan", Copied: done, TotalBlocks: total})
+		_ = out.WriteJSON(CtrlMsg{Type: msgProgress, Phase: "scan", Hashed: done, TotalBlocks: total})
 	}
 }
 
 // sourceLoopParams drives the single read -> hash -> compare -> maybe-send
-// pass over a source path. It is used both by the standalone
-// "source-stream" role (writing to its own stdout) and by a source control
-// agent's push driver (writing into a spawned ssh subprocess's stdin).
+// pass over a source path. The hash of each destination block arrives on
+// Hashes (in ascending index order); the block read for the comparison is
+// the same buffer that gets sent on if it differs — a differing block is
+// never read twice.
 type sourceLoopParams struct {
 	File      *os.File // already opened for reading by the caller
 	Size      int64
 	BlockSize int64
-	Hashes    [][32]byte // previous known hashes, len == block count
+	Align     int64
+	Hashes    <-chan blockHash
 	Out       *FrameWriter
 	OnSkip    func(index uint64)
-	OnSend    func(index uint64, hash [32]byte) // called after the DATA frame is written
+	// OnSend is called for a differing block immediately before its DATA
+	// frame goes on the wire — never after. The push driver relies on this
+	// ordering to record the block as awaiting confirmation before the peer
+	// can possibly ACK it (over a fast local pipe the ACK really can arrive
+	// first), and to apply in-flight backpressure before the send.
+	OnSend func(index uint64, hash [32]byte)
 }
 
 func runSourceLoop(p sourceLoopParams) error {
+	if err := checkBlockAlign(p.BlockSize, p.Align); err != nil {
+		return err
+	}
 	f := p.File
 	buf := make([]byte, p.BlockSize)
-	blockCount := uint64(len(p.Hashes))
-	for index := uint64(0); index < blockCount; index++ {
-		offset := int64(index) * p.BlockSize
-		n, err := readBlockAt(f, buf, offset, p.Size)
+	for bh := range p.Hashes {
+		offset := int64(bh.index) * p.BlockSize
+		n, err := readBlockAt(f, buf, offset, p.Size, p.Align)
 		if err != nil {
-			return fmt.Errorf("read %s at block %d: %w", f.Name(), index, err)
+			return fmt.Errorf("read %s at block %d: %w", f.Name(), bh.index, err)
 		}
 		hash := sha256.Sum256(buf[:n])
-		if hash == p.Hashes[index] {
+		if hash == bh.hash {
 			if p.OnSkip != nil {
-				p.OnSkip(index)
+				p.OnSkip(bh.index)
 			}
 			continue
 		}
-		if err := p.Out.WriteFrame(frameData, encodeDataFrame(index, hash, buf[:n])); err != nil {
-			return fmt.Errorf("send block %d: %w", index, err)
-		}
 		if p.OnSend != nil {
-			p.OnSend(index, hash)
+			p.OnSend(bh.index, hash)
+		}
+		if err := p.Out.WriteFrame(frameData, encodeDataFrame(bh.index, hash, buf[:n])); err != nil {
+			return fmt.Errorf("send block %d: %w", bh.index, err)
 		}
 	}
 	return p.Out.WriteFrame(frameDone, nil)
@@ -116,6 +182,7 @@ func runSourceLoop(p sourceLoopParams) error {
 type destLoopParams struct {
 	File      *os.File // already opened for read/write by the caller
 	BlockSize int64
+	Align     int64 // sector alignment for a raw disk handle, else 0/1
 	In        *FrameReader
 	AckOut    *FrameWriter // optional
 	// OnCtrlMsg handles an interleaved frameCtrlJSON frame (used only in
@@ -145,7 +212,7 @@ func runDestLoop(p destLoopParams) error {
 				}
 				return fmt.Errorf("block %d: hash mismatch after transfer", index)
 			}
-			if _, err := f.WriteAt(block, int64(index)*p.BlockSize); err != nil {
+			if err := writeBlockAt(f, block, int64(index)*p.BlockSize, p.Align); err != nil {
 				if p.AckOut != nil {
 					_ = p.AckOut.WriteFrame(frameErr, encodeErrFrame(index, err.Error()))
 				}
diff --git a/util.go b/util.go
index 643bbed..dd8b3f9 100644
--- a/util.go
+++ b/util.go
@@ -34,6 +34,14 @@ func (b *limitedBuffer) String() string {
 	return string(b.buf)
 }
 
+// roundUp rounds n up to the next multiple of to (to <= 1 is a no-op).
+func roundUp(n, to int64) int64 {
+	if to <= 1 {
+		return n
+	}
+	return (n + to - 1) / to * to
+}
+
 func humanBytes(n int64) string {
 	const unit = 1024
 	if n < unit {