would the DROBO work?
Sounds like the DROBO drive enclosure would suite your needs. It provides 4 disk slots and USB 2.0 interface. You can hot swap any size SATA disks you have around, and it will create a redundant data store (not really RAID) to protect your data from the failure of a single drive. The disks do not have to be the same size, but it helps. Somewhat expensive at $500, but pretty nice.
-howard