File sync solves a persistent everyday problem: keeping the same set of files consistent across two locations — two computers, a computer and an external drive, a local folder and a cloud service. The right tool depends on whether you need one-way mirroring (backup copy), two-way sync (changes on either side get merged), or continuous real-time sync versus scheduled runs. Here are the tools worth knowing about, with honest notes on where each one falls short.
FreeFileSync — Best for Local and Network Sync
FreeFileSync (freefilesync.org) is the most capable free local sync tool available. It handles two-way sync and one-way mirroring, supports scheduling, shows a clear before/after preview of exactly what will change before you confirm, and can sync over local network paths including NAS drives.
Key features:
- Comparison methods: File size and date (fast), file content (thorough, slower). Useful for catching cases where a file's modification date changed but content did not.
- Versioning: Deleted and overwritten files can be moved to a recycle bin or a versioned folder rather than being permanently removed. This prevents sync from propagating accidental deletions.
- Batch jobs: Save sync configurations as batch files and schedule them with Windows Task Scheduler or a desktop shortcut to run them silently on a schedule.
- RealTimeSync: A companion utility included with FreeFileSync that watches a folder for changes and triggers a sync job automatically when files change.
The installer includes a donation prompt and bundled offers — uncheck them during install. The software itself is genuinely free and open-source.
Syncthing — Best for Continuous Sync Between Two Computers
Syncthing (syncthing.net) is a peer-to-peer sync tool that continuously keeps folders in sync between two or more devices — without any cloud service in between. Files go directly from one device to the other over your local network (or encrypted through relay servers if the devices are on different networks).
Syncthing is particularly strong when you want to keep a working folder synchronized between a desktop and a laptop in near real-time. It installs as a background service, provides a browser-based UI for configuration, and is fully open-source. There is no account, no subscription, and no file size limit beyond your disk space.
rclone — Best for Cloud Storage Sync
rclone (rclone.org) is a command-line tool that syncs files to and from more than 70 cloud storage providers — Google Drive, Dropbox, OneDrive, Amazon S3, Backblaze B2, and many others. It is free and open-source.
For users comfortable with a terminal, rclone is extremely powerful. A single command like rclone sync /local/folder gdrive:backup-folder mirrors a local folder to Google Drive. It supports filtering, parallel transfers, bandwidth throttling, and encrypted remotes (store encrypted files in any cloud provider).
For users who want a graphical interface, RcloneBrowser is a third-party GUI frontend for rclone that makes the most common operations accessible without the command line.
Resilio Sync (Free Tier) — Peer-to-Peer with Mobile
Resilio Sync (resilio.com/sync/free) is a commercial product with a permanently free tier that allows syncing between unlimited devices with no folder size limits, with a cap of 10 synced folders. It uses BitTorrent's protocol for efficient peer-to-peer transfers and has mobile apps for iOS and Android, making it useful for syncing files between a PC and a phone directly without a cloud intermediary.
The free tier covers most personal use cases. The paid Home plan adds selective sync, encrypted cloud backups, and server-mode features.
Windows Built-In: Robocopy
Windows includes Robocopy (Robust File Copy), a command-line tool capable of one-way mirroring with options for logging, retrying failed transfers, and excluding file types. It is already installed — no download needed. For a basic backup mirror job:
robocopy C:\Source D:\Backup /MIR /R:3 /W:5/MIR mirrors the source (copies new, updates changed, removes deleted files). /R:3 retries failed files three times. /W:5 waits five seconds between retries. Add /LOG:backup.log to write a log file. Schedule this in Task Scheduler for automated backups.
Choosing the Right Tool
A quick decision guide:
- Local drives or NAS, GUI preferred: FreeFileSync
- Two computers, continuous sync, no cloud: Syncthing
- Sync to cloud storage (any provider): rclone
- PC-to-phone sync: Resilio Sync free tier
- Scripted backup, no install needed: Robocopy (Windows built-in)
One common mistake is treating sync and backup as the same thing. A sync tool propagates changes in both directions — including deletions. If you accidentally delete a folder and your sync tool runs immediately, the deletion propagates to the destination too. For backup purposes, use a one-way mirror with versioning (FreeFileSync's versioning option, or rclone's --backup-dir flag) to retain deleted files rather than permanently removing them.