|The USB port you have the external hard drive plugged into MUST be able to supply 500ma of current.|
You may need to plug it into a different USB port.
If the external hard drive worked fine previously plugged into the same USB port, you may have damaged the data on the drIve by NOT clicking on the Safely Remove Hardware icon and choosing to STOP accessing the drive when you unplugged it while Windows was running.
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Troubleshooting USB device problems including for flash drives, external drives, memory cards.
See Response 1: