I have also posted this to Vista section but if any: My TOSHIBA external HDD stopped working. It failed to copy data and I had to force restart due to system hang up! I found then it isn't even detected. I tried on my friend Laptop XP no success! It just put on green light and shows that there is USB device (at bottom right corner), but nothing!
+1 to Jbennet, if it doesn't work in any computer it has to be an issue with the external itself.
Try pulling it from the enclosure and hooking it up internally so you know if the problem is the drive itself or the enclosure. If you're not comfortable with this anybody who works on computers should be, so ask a friend.