Well sorry to say, but if the PC doesn't recognise it, you may have a problem.
First, you need to try another USB port on your PC, if NONE of them work, then you MUST try this unit on another PC, if it still doesn't work on another PC, then there is either a: A faulty cable, or B: The unit is faulty. Try new cable, if it is not that then it is faulty, these things frequently go faulty for apparently no good reason.
If it does work, then take it back to your PC and go to device manager remove the entire USB controller layer, instructions below, and reboot, when at next desktop try your unit again, it should work. If not, then it simply must be incompatible with your system.
Remove and reinstall all USB controllers To remove and reinstall all USB controllers, follow these steps: Click Start, click Run, type sysdm.cpl in the Open box, and then click OK. Click the Hardware tab. Click the Device Manager button. Expand Universal Serial Bus controllers. Right-click every device under the Universal Serial Bus controllers node, and then click Uninstall to remove them one at a time. Restart the computer, and then reinstall the USB controllers. Plug in the removable USB storage device, and then test to make sure that the issue is resolved.

Usually, external hard drives are plug and play and if ever you need any driver or software, it comes with the drive. Try pulling it out then back again.. While doing that, look at the bottom-right corner of your screen and take note of what the computer says about it.