Have you checked that it is formatted already?
Go to my computer and right click, choose manage.
On the left hand side choose disk management, will take a few seconds but will show you all drives on your computer.
Disk0 is your C:
if you have other drives they will be disk 1 etc but if not the external should be in there as disk 1 (just double check the space of the drive so you know it matches the correct size of the external)
right click the drive then choose format (as you might see the drive but have no drive letter attached to it at the moment as it maybe a raw drive not ntfs).
Let me know what you see.