An overvoltage usually causes the drive's 12V TVS diode to go short circuit. In most cases you can simply remove it.
Here are several photos of typical failures:http://www.users.on.net/
Be aware that many WD external drives incorporate hardware encryption, even if you have not set a password. This means that you cannot just attach these drives directly to your motherboard's SATA port, nor will they work in a third party enclosure. If the small USB-SATA bridge board is also damaged, then you will need to repair it, or replace it with the same board from an identical WD product of the same capacity.