I'd suggest you check if you have the latest firmware update
for your WD Media Player.
While most USB hard drives and flash drives will work with the WD TV Live HD Media Player, we have done the majority of our testing using Western Digital hard drives. As a result we cannot guarantee compatibility with any specific third-party hard drives.
If updating the firmware doesn't help, you can also try the external HDDs after you've disabled the Media Library Compilation on the media player
Hope it helps. Let me know how it goes!