I am right now seeing for the first time an external enclosure that runs off USB power. I knew they were out there, but this is really cool. the Hard disk is a 2.5(or laptop). It doesn't require any external power at all. It's really neat.
You can get the enclosure for as low as like 10USD. The hard drives are a little up there, but for about 75USD you can get a decent 60 GB.
IT may be a bit bigger than a flash drive, but if it's storage size and portability you are going for, it's a great solution. Just toss it in you laptop bag. Throw it in with your repair kit and you have yourself a decent backup system for those onsite calls. For faster data transfer, you can get a 2.5 to Internal IDE adapter and hook it straight in to your motherboard. The adapters are only a couple dollars.
If your not into the DIY, I'm sure there's a pre-assembled one out there. I think I have a 10Gig drive at home, I may end up ordering one myself.
["enclusure" => "enclosure" - Spock]