I was almost sure you would say Western Digital, because this has already happened to me (not exactly the same situation, but very similar) and I have many friends that have had many issues similar to this one with Western Digital hard drives, especially with older Caviar hard drives. When it happened to me I was lucky I was waiting for a new hard drive, because the Western Digital was useless. Why this happened to the hard drive was because first of all the hard drive was old and also because it was broken already. I am not sure though this is the same situation as yours. If it is then I am sorry to inform you, but that hard drive is pretty useless. Maybe start checking for cheap new ones?
dan