Another possible solution :

Symptoms:

USB shows in the device manager (right click on my computer and click manage, devices) either "Unknown USB device" or "name of USB hard drive manufacturer"  both of which have the yellow ! next to them saying they aren't installed correctly.

Resolution :

Right click on the device in the device manager that shows it isn't installed, click update driver software, enter this location assuming your C drive is your main drive

"C:\Windows\winsxs" and ensure the box is checked to include subfolders and hit ok.  It should reinstall immediately and show up as a new drive in "My Computer"

You may have to uninstall the device first, replug it back in and then run through the above steps.