Try another usb port. If you're using a hub, try connecting it directly to a usb port on the computer itself. Run Seatools for windows on the drive to check for problems. Run the short DST and long generic. If it won't run in windows due to the drive disappearing, run Seatools for DOS.
Is this an actual external drive or are you using a drive with an added enclosure? If so, could be the enclosure itself or a lack of power to the drive if it's supposed to have it's own power source plugged in.
Have you tried other external drives to see if the problem is related to the drive or the usb system?
Hey i have tried it on a friends computer and it didn't work. It doesnt have its own power source. I tried Seatools for windows and it didnt show up so i tried Seatools for DOS but because i dont have a cd player i cant use it. Any other ideas? Thanks heaps.