haha, well you'd only need one, at least to start, and I've found that Target has good deals on external storage from time to time. Plus, if performance isn't too important, then the speed of USB should be fine, especially if it's a system you're willing to let do its backup thing overnight.
In particular, this is actually one of the things that makes tapes more desirable than hard disks. (Although I'd still consider having rotating single disks better than using RAID at all in a backup system, at least in a desktop-level backup system. It's different of course if you're building a larger d2d2t system.)
In general with both DAT and LTO, you shouldn't need to worry about having a specific mechanism. (like, your specific serial number) -- but you do need to have a type of mechanism that is compatible with reading your tapes. LTO is write-compatible one generation backward and read-compatible two generations backward. DAT has a similar system but it's slightly more erratic.
In particular, this is actually one of the things that makes tapes more desirable than hard disks. (Although I'd still consider having rotating single disks better than using RAID at all in a backup system, at least in a desktop-level backup system. It's different of course if you're building a larger d2d2t system.)
In RAID, or as independent disks but with the data duplicated, say using Retrospect or just by dragging and dropping the files? (or, even better, using drive "A" one week and "B" the next, for a little bit of version history?)