I agree with Steve. I have a 160 gig hard drive in an IDE t USB 2.0 case that I hauled arond for school as it was cheaper than the 512 flash drives at the time and obviously had more capacity (though it was less convenient). I never had any trouble with that thing flopping around in my solid axle swapped yota, being exposed to well below zero temps, etc. It performed better than the computer I hooked it to in my apartment....
For the record, it was a Western Digital HDD, but I'm not sure how much of a difference brand makes. My point is that I think that it would be fine if you were to follow Steve's instructions. I don't think you'll have any problems and even if you do I'm sure it will be archieval in nature anyway so you'll have another copy of the data elsewhere.
The only other option I see is backing everything up on tape when stopped, or buying $$$ worth of flash drives