To access Disk Management, right click on My Computer, and select Manage from the context menu – this will open the Computer Management console (alternatively, Disk Management may be accessed from Control Panel> Administrative Tools> Computer Management).
From the Computer Management console, click on Disk Management which is located under the Storage section in the left hand pane.
Can you see your external drive in disk management?
You should be able to right click and select format.
Once formatted the drive should apper in My computer.