Click start control panel administrive tools ,computer management ,device manager look through all of your devices if you see a yellow question mark?or exclamation mark ! or red x
right click to reinstall drivers or
if you can see your usb but its not working
ports(com&lpt)right click update driver
make sure the lead/s that attach to this drive have a secure seating and are not bad/faulty the light on only tells that its getting power it might not be getting enough power you might test the lead/s or
just replace the cable of the hard disk, when you plugged the external disk drive you got that windows could recognize the USB device change the cable everything should work fine again, sometimes the wires inside the cable will be damaged due to bending or stretching or placing heavy equipment upon the cable,
check the USB leads that attach to the motherboard usually red-white-green-black make sure they are securely seated and have no dust build up on them dust will cause static and a lot of unforeseen problems
hope this helps