|The USB port the external hard drive is plugged into must be able to supply 500ma of current. On some laptops, if the external drive is 2.5", you must plug the external hard drive into TWO USB ports that are built into the laptop. |
They often will NOT work properly when plugged into a USB hub.
If the USB cable you are using did NOT come with the external drive, it may not be adequate.
If you have NOT been clicking on the Safely Remove Hardware icon and STOPping accessing the drive when you unplug it while Windows is running, the data on the drive may be damaged - usually that can be fixed by using certain programs (including Recuva and Testdisk).
Troubleshooting USB device problems including for flash drives, external drives, memory cards.
See Response 1: