I looked at the WD site for info that might help. You can try also since you can give the specific model number of your external drive to narrow things down.
However, you might start with this. WD has a diagnostic program and according to the URL (copied and pasted):
The Windows version of the Data Lifeguard Diagnostics utility can perform drive identification, diagnostics, and repairs on a Western Digital FireWire, EIDE, Serial ATA, or USB drive. In addition, it can provide the drive's serial and model numbers.
Here's the URL which also contains a how to video
http://wdc.custhelp.com/app/answers/detail/search/1/a_id/940/c/130/p/228,209,316Good luck!