|If you HAVE NOT dropped the drive, that probably indicates the USB port you have it plugged into cannot supply 500 ma of current to the drive. |
Troubleshooting USB device problems including for flash drives, external drives, external memory card readers.
See Response 1:
In addition to the info there....
For some desktop mboards you can have problems when an external hard drive is plugged into one of a pair of USB ports on the back of the case that are connected to the same USB controller, if there is another device plugged into the other USB port for the pair, because the pair can't actually supply 500 ma per port - they can only supply 500 ma in total. Usually those pairs of ports are one above the other, not beside each other. If you have two devices plugged into such a pair of ports, try plugging one of them in elsewhere on the back of the computer.
For MANY laptop and netbook mboards similar applies. The built in USB ports often cannot actually supply 500ma per port - they supply 500 ma in total for two ports that are close to each other. In that case, the external drive should work fine if it's the ony thing plugged into the USB ports built into the laptop or netbook, or if you have more than two ports, the only thing plugged into two ports close to each other.