What's wrong with getting a simple USB external drive? If your I/O is going to be USB 2.0 anyway, why is this docking thing any better?
Something else you might consider is getting a much-larger internal SATA drive, then ghosting one of your current drives to it, and swapping it in for the ghosted drive. If done correctly, the result will be the same system you already have plus a ton more room on one of the drives. Then you can drop the swapped-out drive into an external enclosure, or put it in a drawer as a backup.