Have a quick check to see if they show up in Disk Management. Click Start, right-click on "Computer" and click "Manage", then go into "Disk Management" on the left.
In Disk Management, you should see two drives - Disk 0 and Disk 1. If either shows 'Offline', right-click on it and set it to online. If the WD drive is already online, try right-clicking on a partition and set/change the drive letter. If you have any network drives, make sure the drive letter(s) don't conflict, otherwise the network drive letter will hide the hard disk drive letter.