when you plug in the external harddrive, do you hear
1. any sort of clicking noise,
2. or sound that sounds like the harddrive is spinning then stops then spins and stops kind of cycle.
3. or nothing at all
if you do there is a mechanical failure for your harddrive.
if you hear continuous spinning then perhaps the usb cable or the interface for the external harddrive is damaged some how and you may need to bring in your external harddrive to a pc-repair shop