Syncing two external hard drives

Hi, quick question please: I have an identical set of data on two external hard drives (data not on my PC) that I want to keep synced. These are drives I won't have plugged in all the time (as in powered up).

I've set up a new sync for them, but I was wondering what happens if I change data in one drive while the other one is not switched on. If I've selected automatic sync, will they sync the next time the two drives are plugged in? Will ATI warn me that I must plug in the second drive? Should I just leave automatic sync off and manually sync?

I guess I'd also appreciate confirmation that this is the best way for me to keep a backup of all my data on one drive. Initially, I tried creating a backup, but as the data set was already identical on both drives, the process soon quit, saying there wasn't enough room. I realized then that it wanted to create another full copy of the drive, not just scan the other one and sync them. That's why I went to sync.

Thanks for the pointers when you have a chance.

Lucas