How do you sync a Mac and an external drive in both directions?

By Kayabyte · Last updated 13 November 2026

Short answer: Two-way sync means a file changed on either side is copied to the other. The hard part is deletions: if a file is missing on one side, the tool must know whether it was deleted there or never existed. Finder and plain rsync cannot know. A two-way app remembers the state after the last sync. Pairfold does this: it notices a deletion because the file was in the last-synced state, archives the other copy instead of deleting it, and leaves a file that both sides changed for you to decide.

What the free tools do

Finder: copy or merge by hand, in one direction at a time. No comparison, no memory of the last sync.

rsync: one direction. Running it both ways with -u (skip files that are newer on the destination) catches simple edits. Deleted files come back from the other side, and a file edited on both sides keeps the newer copy and silently loses the other.

The cases that decide whether you can trust a two-way sync

Edited on one side only: copy it over. Every tool does this.
Deleted on one side, unchanged on the other: the deletion should reach the other side.
Deleted on one side, changed on the other: the change should win. Pairfold keeps the changed file and restores it.
Edited on both sides: a conflict. Pairfold lists it in the preview and by default leaves both alone; you can choose to let the newer one win, and the older one goes to the archive.
New on both sides with the same name: also a conflict.

We ran each of these cases on an exFAT drive in our own tests. After the changes a third run did nothing, the two trees matched, and every replaced or removed item was in the archive.

With Pairfold

Choose Two-way for the pair. The preview shows each action grouped as New, Changed, To archive and Conflicts. Nothing is deleted outright: removed or replaced files are moved to the archive folder. The state after the last clean run is what lets Pairfold tell a deletion from a new file.

Limits: it works on pairs of folders, and a run that would replace or archive a large share of your files waits for your confirmation. If the first run is a pair that has never been synced before, there is no memory yet, so Pairfold treats every difference as new or changed and asks about real conflicts. macOS 26 or later.

Steps with Pairfold

New Pair: choose the folder on your Mac and the folder on the drive.
Choose Two-way.
Check and Preview. Read the Conflicts group first.
Click Sync. Later runs can be scheduled or start when the drive is connected.

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

Questions

What happens if I edit the same file on both sides? It is a conflict. Pairfold leaves it for you to decide, or, if you choose, keeps the newer one and archives the older one.

Will a deletion on the Mac delete the file on the drive? The drive’s copy is moved to the archive, not deleted.

Is two-way a good idea for a backup? Usually not: a mistake on one side reaches the other. For backup use Mirror, see mirror without deleting files.

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.