Hello indoow and welcome to Seven Forums. Couple of things to try:
1. Try using a different USB port.
2. Try changing the drive letter. Drive Letter - Add, Change, or Remove in Windows 7
3. If you have access to a live Linux CD like Ubuntu you may find that Ubuntu will recognize the external hard drive in Computer.
4. The hard drive may be going bad. You could go to the manufacturer's web site and look for diagnostic tools to check the drive. Or you could use SeaTools for Windows from Seagate (works on non-Seagate drives as well.) | Seagate