Is the WD my book an external HDD?.(see last question)
If the drive shows in My Computer can you run CHKDSK or Defrag?.
Have you checked Event Viewer for any error messages?.
Is the listed in device manager?.
If its and external HDD,I have known the USB interface to fail.If its at all possible(seek advice first),can you remove it from the caddy and use as an internal drive?.
Just be careful,doing even the basic procedure could cause data loss.