Synchronize two working external drives over network 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 &lt;user&gt;@computer.2:/path/to/drive.2 /path/to/drive.1/

# sync drive.1 to drive.2
rsync -av --stats &lt;user&gt;@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.