We have two designers both using iMacs, running macOS Sierra; each have their own 4TB USB hdd. The designers have to transfer files to each other frequently over AirDrop but this is tedious.

What would be the best way to have the drives automatically sync files on both drives?

We've done some searching but most of the scenarios are only one working drive syncing to a back up. In our case both drives are being used and files are being created and edited on each. Essentially I would like something like a synchronized ftp or a remote source control, something that automatically updates files that are changed on either drive and creates/deletes files. Bonus points for adding a third external into the mix that is used just to have a offline backup of all files.


This ServerFault entry asks a similar question. The answers there are rsync and a product called Unison. It sounds like either solution fits the problem you have in front of you.

  • For rsync, something like the following recipe would likely work (I haven't run this, so the syntax may need some fiddling):

    # sync drive.2 to drive.1
    rsync -av --stats <user>@computer.2:/path/to/drive.2 /path/to/drive.1/
    # sync drive.1 to drive.2
    rsync -av --stats <user>@computer.1:/path/to/drive.1 /path/to/drive.2/
  • And Unison, the other option from the ServerFault entry, looks like it's a software package designed specifically to solve your type of synchronization problem.

  • haha genius! much obliged! – luggage chariot Dec 15 '16 at 18:31
  • 1
    Keep in mind these commands need to be run each time to bring the drives in sync with each other. Tools such as SyncThing, Dropbox, and ResilioSync automate the task in real-time. – Harv Dec 18 '16 at 15:06

Syncthing is an open-source solution to synchronise files across networks.

It synchronises files either way, you can add as many nodes as you like, it allows you to keep version history in case some changes get overwritten.

It works in background, offers a web management interface, and handles well situations in which you disconnect one or more drives.

You can install with Homebrew:

brew install syncthing

By default it enables the client interface at the address

Syncthing interface


Edit for at least Mojave and later - file sharing, caching, and being a Time Machine remote destination are now all possible without needing the Server.app software from apple, all within the "Sharing" preferences within System Preferences.

This doesn't directly answer the question as posed, but might be a different way of getting the results you want. If you install macOS Server from Apple onto one (or both) of the iMacs you could use file sharing to just share a folder to both users, and not worry about the synchronization at all. Gig ethernet makes for pretty reasonable file transfer speeds.


You could then also use the TimeMachine function of the server software to provide for an additional backup destination for the iMacs, and the caching feature to speed up any update/iCloud/Apple traffic on your network. Well worth the $20 for the server software.



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