Merge one huge folder into another, fast, with many files copying at once and resume after any interruption.
Built for moving terabytes between NAS shares without babysitting the copy.
Dragging a folder onto another in Finder replaces matching folders rather than merging them. It copies one file
at a time, and if anything goes wrong 9 TB into a 14 TB copy, you start over. rsync merges correctly, but it is
single-threaded and leaves you staring at a terminal.
SpeedMerge does one job well. It takes Folder A and merges everything inside it into Folder B, keeping the exact directory structure:
Folder-A/Media/TV/Show/Season 01/Episode 01.mkv → Folder-B/Media/TV/Show/Season 01/Episode 01.mkv
Files already in the destination are left alone, new and changed files are copied in, and the source is never modified. Many files copy at the same time, so fast networks and multi-disk arrays stay busy. Every copy can be resumed after a stalled share, a dropped network, a crash or a reboot.
- Parallel copies. Choose 1–32 simultaneous file copies (default 8), and change the number while a merge is running to find the best setting for your hardware.
- Parallel scanning. Several scanner threads walk the source tree at the same time, so copying starts right away, even on folders with millions of files.
- Built for large files on network shares. It uses direct
pread/pwritein 8 MB chunks withF_NOCACHE, so multi-gigabyte files don't fill the Mac's memory cache or slow down the copies running beside them. - Skips work that is already done. A destination file with the same size and modification date as the source counts as up to date and isn't copied again.
- Copy only. The source folder is only ever read: nothing is moved, renamed or deleted.
- No half-written files. Each file is written to a hidden
.<name>.<hash>.smpartialfile next to its final location. It is renamed into place only when complete, so the destination never holds a truncated file under the real name. - Overwrite when different. A file at the same path that differs in size or date is replaced, and the copy gets the source's modification date.
- Won't write to the wrong disk. If a share disappears, macOS can leave an empty local folder at its old mount point. SpeedMerge checks it is still writing to the same volume, and it never creates the destination folder itself, so a dropped share can't silently fill your boot drive.
- Resumes after a crash or quit. The job is saved when it starts. Next launch, a banner offers to Resume: finished files are skipped, and partial files continue where they stopped. The last 64 MB is copied again to be safe.
- Stall watchdog. A file that makes no progress for 60 seconds is abandoned and retried, up to 3 attempts with increasing delays. A hung network read can't freeze the whole merge.
- Share-drop detection. Both shares are checked every few seconds. If one goes offline, everything pauses with a clear waiting for the share status and continues on its own once it is back, even if it remounts as a new device. Errors caused by an outage don't use up a file's retry attempts.
- Keeps the Mac awake for the length of the merge. The display can still sleep.
- An overall progress bar with bytes done, live speed (10-second average) and time remaining.
- Counters for copied, already up to date, found, retried and failed files.
- A live list of the files being copied right now, each with its own progress bar.
- Every failure is listed with its reason, and running the merge again retries only those files.
- A full timestamped log of each run in
~/Library/Logs/SpeedMerge/. - Pause / Resume / Cancel at any time. Cancelling keeps partial files, so starting again picks up where it stopped.
These are never copied: .DS_Store, .Trashes, .Spotlight-V100, .fseventsd, .TemporaryItems, and the common
Synology / QNAP system folders @eaDir, #recycle, #snapshot, @Recycle, @Recently-Snapshot, .@__thumb.
| Choose two folders | Merge in progress |
|---|---|
 |
|
| Finished | Resume after a crash or quit |
 |
 |
- Download
SpeedMerge-x.y.z.dmgfrom the latest release. - Open it and drag SpeedMerge into Applications.
- The first time you open it, right-click the app → Open → Open. The app is ad-hoc signed and not notarized by
Apple, so Gatekeeper asks once. Alternatively, run:
xattr -dr com.apple.quarantine /Applications/SpeedMerge.app
Requires macOS 14 Sonoma or later and runs natively on Apple silicon and Intel Macs.
- Copy from: drag the source folder onto the left box (or click Choose…).
- Merge into: drag the destination folder onto the right box.
- Set Parallel copies (8 is a good start) and press Start Merge.
Network shares work exactly like local folders. Connect to them in Finder first (⌘K), then drag them in from the
sidebar or from /Volumes.
If you quit during a merge, SpeedMerge asks first, then stops cleanly so the merge can be resumed. If the Mac crashes or loses power, the next launch shows the resume banner.
The same engine runs without the window, which is useful over SSH or in scripts:
/Applications/SpeedMerge.app/Contents/MacOS/SpeedMerge --cli <source> <destination> [--parallel N]Log: ~/Library/Logs/SpeedMerge/SpeedMerge-2026-01-15_09-30-00.log
12.4% 412.6 GB of 3.31 TB 498.2 MB/s copied 1830 skipped 9214 failed 0 [running]
It prints progress every 2 seconds and exits with 0 when every file was merged, 1 if some failed and 2 for a
usage error.
On a fast network, the drives are usually the limit, not SpeedMerge. A few things worth checking:
| Check | Why it matters |
|---|---|
Link speed: run networksetup -getMedia Ethernet and look for 10Gbase-T |
A bad cable or switch port can silently negotiate 1 GbE or 2.5 GbE. |
| VPN / zero-trust network clients | Some route LAN traffic through the tunnel, which can cut throughput by 5–10×. Turn them off or exclude the NAS subnets. |
| Parallel copies | Spinning-disk RAID arrays seek between files. Try 4, 8 and 12 and watch the speed. There is usually a sweet spot, and going past it adds disk latency without adding throughput. |
| File size | Large video files run close to wire or array speed. Folders of small photos are limited by per-file overhead and disk seeks. |
The parallel-copies stepper works live, so you can tune it during a long merge without restarting.
scanner threads (4) ──► bounded file queue ──► copy workers (N) ──► .name.hash.smpartial ──► rename into place
│ ▲ │
└─ mirror folder tree, index destination │ └─ set source modification date
│
watchdog ── stalled copy (60 s) ──► retry ─────────────┘
network monitor ── share gone ──► pause all ──► share back ──► continue
- Engine:
Sources/SpeedMergeCoreis a UI-independent library. Plain POSIX I/O runs on dedicated threads coordinated by a single condition variable, with no third-party dependencies. - Partial-file names include a hash of the source size and modification date. A partial file from an older version of the source is never resumed into a newer one.
- Up to date means same size and modification time within 2 seconds, which covers the timestamp rounding of SMB, exFAT and FAT.
- Symbolic links are recreated as links. Empty folders are created. File permissions, extended attributes and Finder tags aren't copied: the destination share's own permission rules apply.
Requires Xcode 16 or later (Swift 6 toolchain).
git clone https://github.com/bytePatrol/SpeedMerge.git
cd SpeedMerge
./build.sh # universal SpeedMerge.app in the project folder
./build.sh --dmg # also creates dist/SpeedMerge-<version>.dmg and .zip
swift test # engine test suiteThe test suite covers nested merges, skip and overwrite rules, modification-date preservation, resuming from partial files, ignoring stale partials, stall recovery, persistent failures, cancel-and-resume, pause handling, exclusions and a 400-file parallel run.
Sources/
SpeedMergeCore/ merge engine, job store, logger (no UI code)
SpeedMerge/ SwiftUI app + --cli entry point
Tests/
SpeedMergeCoreTests/ engine tests
Resources/Info.plist app bundle metadata
scripts/make-icon.swift draws the app icon at build time
build.sh builds, signs and packages the app
Does it delete anything from the destination? No. It only adds files and replaces files at the same path that differ. Nothing missing from the source is removed, so it's a merge, not a mirror.
What happens if I run it twice? The second run checks every file, finds them up to date, and copies nothing. That makes re-running a cheap way to check a finished merge or retry failures.
Can I copy NAS-to-NAS without going through the Mac?
SpeedMerge runs on the Mac, so data passes through it. On a 10 GbE network that usually isn't the bottleneck,
because the drives are. If you need to skip the Mac entirely, run rsync on one of the NAS devices instead.
Where are the logs and the saved job?
Logs are in ~/Library/Logs/SpeedMerge/. The resumable job is in ~/Library/Application Support/SpeedMerge/last-job.json.
SpeedMerge is free software under the GNU General Public License v3.0.