|Make sure it's plugged into a USB port that can actually supply 500ma (or two of them in some cases). If the USB cable did not come with the drive it may not be adequate.|
The drive may not work properly or be detected properly when plugged into an external Hub.
Troubleshooting USB device problems including for flash drives, external drives, memory cards.
See Response 1:
Examine the USB port and the plugs on the USB cable you're using to to see whether they're damaged.
E.g. Bent - such that the fit of the plug is loose - outer metal shell in the port, squished metal outer shell on a plug; bent or loose contacts in the port or plug, the plastic blocking plug that prevents you from plugging in a USB plug upside down is loose or missing.