For the second time today, I call Linux as the solution.
Running Linux via a LiveCD will let you plug in the HDD and see if the result is just Windows playing up or if it's the drive or enclosure.
If you're considering this, let me know and I'll guide you through it. The other guy I helped today I set up with Slax (
http://www.slax.org/), nice and small and does the job.