Could you tell us which WD external drive you have?
Click on this link
http://support.wdc.com/product/download.asp?lang=en
And which computer you are using:
PowerPC, Imac, MacBook or Macbook Pro?
What Operating System you are running?
Tiger 10.4, Leopard 10.5 or Snow Leopard 10.6
How do you connect to the computer?
Via USB cable, Firewire 1394 cable