Most external USB drives are a internal with a powersupply and controller. If possible (and as a last resort) you could remove the drive the enclosure and connect it directly to a SATA/IDE port and see if it is recognized that way.
Tell us a little more about your setup, computer make, model#, OS, hard drives in the computer, and if everything is working properly. Then please give us more information about your WD external HDD, model#, size, if it has both a USB and eSata connector port, if it has been running properly, & if you switched it from a Win-XP to a Win-7 computer recently? When was the last time the external drive worked properly?
The more info we have, the easier it is to resolve the problem.
That way someone will be able to get you running again.
If it's there, right-click and remove any partition that may be there (if there is no data on the drive). Create a new one and format it NTFS with the default options. You can change the drive letter, too, if you don't like what you get.