Unplug the drive from your computer, then restart your comp.
On the drive there is a round port for DC input, plug the second USB cable into that and then into your comp. AFTER that, you can plug in the attached USB cable and see if the drive appears.
If it appears: The drive is not getting enough power through one USB port, therefore needs to be plugged into two.
If it doesn't appear:
Go Start>Run> type in "diskmgmt.msc" and click OK
See if you find this drive listed as "Disk 1", or "Disk 2", etc.
Also...
What Operating System is on your Computer?
Have you tried other computers?