FreeFileSync

Last reviewed Sep 13, 2026 · 749 words

FreeFileSync is the tool for the job Syncthing refuses to do: show you, before anything is copied, exactly which of the 40,000 files in a folder pair differ and in which direction, then let you fix the three you disagree with before pressing Synchronize. It is a desktop program for Windows, macOS and Linux, not a service, and it has been quietly excellent at that one job since 2008. The 1,634-star GitHub repository is an unofficial mirror of the source tarballs the author publishes at freefilesync.org, which is where you actually download it.

Four sync modes, and only one of them is safe to run blind

Compare a left and right folder and FreeFileSync classifies every file: only-left, only-right, left-newer, right-newer, conflict. Then you choose a variant. Two way propagates changes in both directions and uses a database file in each folder to tell a deletion from a new file, which is what makes it safe to run repeatedly. Mirror makes the right side identical to the left, deletions included. Update copies new and newer files left to right and never deletes. Custom lets you set the action per category. Mirror is the one that bites: run it with left and right swapped and it will faithfully delete your originals, which is why the visual preview is not decoration. Read the grid before the first run of any new job, every time.

Versioning turns a mirror into something you can undo

Under deletion handling, choose Versioning and give it a folder, and every file the sync would delete or overwrite is moved there with a timestamp instead of vanishing. That single setting is the difference between "my mirror copied my mistake to the NAS" and "my mistake is in the versions folder". It is still not a backup. A mirror has no history, no deduplication and no encryption; if you want to restore last Tuesday's state of a folder, that is a job for restic or Duplicati, and the 3-2-1 backup guide explains why a sync copy counts as at most one of the three.

Scheduling: save a batch job and hand it to the OS

Configure a folder pair and its variant, then File, Save as batch job, producing a .ffs_batch file. Run that file from Task Scheduler on Windows, launchd on macOS or cron on Linux, and set it to run minimized with error handling set to ignore or to pop up, as you prefer. The companion program RealTimeSync, bundled in the same install, watches folders for changes and fires the batch job after a quiet period, which gives you near-continuous mirroring to a NAS without a daemon. Network paths work as plain SMB mounts; SFTP, FTP and Google Drive are built in.

The Donation Edition question

The free build is complete. Donating gets you the Donation Edition, which adds parallel file operations, silent auto-updates and a portable build. The installer's nag screens and bundled offers are the catalogue's "donation prompts" con; read each installer page rather than clicking Next. For a home NAS mirror over gigabit, parallel operations rarely matter; for hundreds of thousands of small files over SFTP they cut runtime noticeably. Decide after your first big job, not before.

Where Syncthing and rsync fit instead

Syncthing is a continuous, multi-device, headless mesh, and the right answer when three machines must stay in step without anyone looking. rsync is the right answer inside scripts on servers with no display. FreeFileSync is for a human with a screen who wants to see a diff, run a mirror on demand or on a schedule from a desktop, and never install anything on the far end. The Nextcloud vs Syncthing comparison covers the always-on options; FreeFileSync is the deliberate, visible one that the file-sync category otherwise lacks.

What I'd do

Use FreeFileSync from the desktop for the folders I curate by hand, photos and documents, mirroring to the NAS with Versioning on and a batch job that RealTimeSync triggers. Keep Syncthing for the working directories that must follow me between machines. Run restic against the NAS nightly so the mirror is a convenience layer on top of a real backup, never a substitute for one. And check the preview grid every time I change a job, because the tool will do exactly what the arrows say.