If it's a USB hard drive, it's possible that it's only seeing the USB controller and not the drive. Just a few questions,
1. Is the drive spinning and/or making any strange noises?
2. Does the computer detect the drive's model number or just a generic controller?
3. Does the drive appear and show capacity in disk management? (found in control panel > admin tools > computer management > disk management).
This will really help to know what's going on.