Kayabyte

How do you sync a folder to an external drive automatically on a Mac?

By Kayabyte · Last updated 13 November 2026

Short answer: macOS has no setting that copies a folder to a drive by itself. The free routes are Time Machine (backs up the whole Mac, or everything except what you exclude, not one chosen folder), a launchd job that runs rsync, or an Automator or Shortcuts workflow. They work, but you write and maintain them, and when the drive is missing they can fail without telling you. Pairfold lets you pick the folder and the drive, set a schedule or “run when the drive is connected”, and shows what happened in a history list. macOS 26 or later; see the app page for price and where to get it.

Method 1: Time Machine

Time Machine backs up your Mac to a disk every hour while the disk is connected. You can exclude folders, but you cannot tell it to back up only one. It keeps older versions, which a plain copy does not. If what you want is “my whole Mac, with history”, use it. If you want “this project folder on that drive, readable in Finder without Time Machine”, it is the wrong tool. More in Time Machine vs folder sync.

Method 2: rsync with launchd

  1. Test the command by hand first: rsync -avn ~/Projects/ /Volumes/Backup/Projects/ (-n is a dry run).
  2. Save a script that runs the same command without -n. Add a check that the drive is there, for example [ -d /Volumes/Backup ] || exit 1. Without that check, rsync may write into a plain folder on your Mac’s own disk named /Volumes/Backup.
  3. Put a launchd job in ~/Library/LaunchAgents that runs the script. launchd can start a job at an interval (StartInterval), at set times (StartCalendarInterval) or when a volume mounts (StartOnMount).
  4. Load it with launchctl.

Things that go wrong: macOS may ask for permission to use removable volumes or Documents, a job that has no permission fails quietly, and a typo in a plist gives no message. There is no preview and no history unless you add a log file.

Method 3: Automator or Shortcuts

Both can run a shell script on a schedule or when something happens. It is the same copy as Method 2, in a friendlier wrapper.

With Pairfold

Pairfold checks before every run that both folders are the ones you chose. If the drive is missing, or a different disk sits at the same place, the run stops and says so; a missing disk is never read as an empty folder. Each file is copied under a temporary name and checked, so unplugging the drive in the middle leaves the old file as it was. The next run cleans up and continues.

Schedules and “run when a disk or share is connected” work while Pairfold is open in the menu bar. Opening Pairfold at login is optional and off until you turn it on.

Limits: not a bootable clone, no version browser (the archive is a plain folder), macOS 26 or later.

Steps with Pairfold

  1. Click New Pair and choose your folder and a folder on the drive.
  2. Choose Mirror. The drive’s folder will follow yours and yours is never changed.
  3. Click Check and Preview, read the list, then Sync for the first run.
  4. Open the pair’s settings. Choose a schedule, tick Run when a disk or share is connected, or both.
  5. In Pairfold’s settings, turn on Open Pairfold at login if you want it to keep running after a restart.

Get Pairfold: see the app page for where to get it

Questions

Can I back up one folder with Time Machine? Not by choosing it alone. You can exclude everything else, which is awkward.

What happens if the drive is not plugged in at the scheduled time? Pairfold does not run that pair and says the disk is not connected. Plug it in and run again, or let run-on-connect do it.

Will it keep running after a restart? Only if you turn on Open Pairfold at login.

Does Pairfold work with exFAT drives? Yes. APFS, Mac OS Extended, exFAT and FAT32 all work. See why files copy again on exFAT and FAT32.

Apple, Mac, macOS, Finder, Time Machine, and iCloud are trademarks of Apple Inc. Pairfold is made by Kayabyte and is not affiliated with or endorsed by Apple.