My external HD is a Seagate FreeAgent GoFlex Desk 2TB drive formated with Mac OS (Journaled). It's networked into my 3 TB Time Capsule through the USB port on the Time Capsule. I'm using two MacBook Pros, both running Mac OS X Lion.
I want to use the Seagate drive as a primary storage for documents so they are easily accessible from both MacBook Pros. Since that's where most of our data will be stored, I want to back it up. Using Time Machine to do that seems like the easiest option, if it can be so configured.
Pro, Mac OS X (10.7.3), Mac Time Capsule, Seagate GoFlex
You will have to plug the external drive into a MBP and use TM from there. Or use a 3rd party utility of which there are plenty of good choices.. SuperDuper, CCC, Chronosync are some I have come across.