USB drives have to be reformatted with NTFS, or large backups fail due to their file size. I routinely backup 20 gig images to 2.5 inch USB drives. They get rotated offsite just as you would with tapes.
Thanks for the tip, I'll have to check to see if we're formatting NTFS but I don't think so since the Acronis doesn't seem to have the problem. Backing up local does help the one main problem the image backups seem to have - corruption of the image during backup due to network issues. Doesn't happen much but something to be aware of. But, if you're backing up local, and disconnecting those drives for archive you should be in good shape.