20GB USB drive - This is just one of hundreds of examples. This one gets its power via the USB connector. If you get a larger one I've even seen people create a power supply to feed the appropriate voltge to the drive. Plus these drives are mobile drives with additional shock protection. I'm sure there's a chance of problems but I wouldn't worry about them.
You need to find the max size that your head unit will accept. For example, the new Pioneers can only handle 5000 songs in 400 folders. There was a guy at one of the car audio forums that found this limit by connecting a 60GB drive.
-Robert