By Chris Hoffman at MakeUseOf.com…
External drives — either USB flash drives or external hard drives — should be easy to use. In some cases, you may connect your drive to a Windows PC or another device with a USB port and find that it’s not recognized. This problem can be caused by partition issues on your external drive, using the wrong file system, dead USB ports, driver issues in Windows, or other problems. In a worst case scenario, the drive itself may simply be dead.
via External Drive Not Recognized? This Is How To Fix It In Windows.