How do I get two hard drives that mirror each other, even if they arent always connected?

I have a 2TB external drive and a 2TB internal drive. I use them both for video editing, but I work with others. Is it possible that when I connect the two drives, they update to look like each other so that I can disconnect the external and give it to another editor to connect remotely? Would that be using RAID or something else?

Its kind of like a giant 2TB dropbox. Syncing when they're put together.