Basically you want to create a script that gets triggered on the local machine when the drive gets connected, runs some commands via SSH to assure that the sync can run safely (pointer: AIO Borg Backup + Crashplan - #13 by szaimen) and sync the backup archive to the local machine.
However I fear since most of this is linux knowledge and not really AIO related, I don’t have the time to help you here. Hope that someone else steps in to help you on this and Google is your best friend 