We have a 12TB FreeNAS box in the office, and are looking for a way to keep a backup of it offsite.

We're considering (1) tape; (2) a bunch of bare drives (popped into a spare hotswap bay); (3) external drives. Any advice on which solution is best? (Online backup is not an option because our internet connection is too slow.)

And, is there some software that will keep track of which files have been backed up and which haven't? So that when one backup unit fills up, we can continue the backup on the next? (We don't want to have to back up to a 12TB device.)

This software could run, preferably, on the NAS itself; or from one of our Mac clients.

Our goal is a situation where

1. we attach some backup device;
2. it automatically fills up with stuff from the server; the contents of this unit are catalogued somewhere
3. something prompts us to replace with a fresh drive/tape;
4. backup continues until full, including any files that have changed since being backed up.