Is there someplace that will do a virus scan of the drive cheap? I wouldn't plug it into a friend's computer to test it because if there's a virus on it they may not be a friend much longer.
I'd say the possibilities are:
a) something physical in the drive. They are not the most rugged. Electronic stuff may be intermittently connected.
b) the drive picked up a virus
c) the USB port or system you are plugging into is flaky.
First thing I'd do is try to get it tested for virus at a PC store and while they are at it maybe run some other tests, that is if the place doesn't charge a lot for that service.
Once you rule out the virus then you may try some diagnostics like plugging it into a different PC.