Synchronize two working external drives over network

Solution 1:

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.

Solution 2:

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 http://127.0.0.1:8384:

Solution 3:

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.

http://www.apple.com/macos/server/

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.

http://www.macworld.com/article/2918973/save-your-bandwidth-by-using-a-caching-server.html

http://www.macworld.com/article/3130110/servers/turn-up-the-time-machine-service-in-macos-server.html