External HDD unmounting unexpectedly
This is symptomatic of a hardware failure in most cases. You'll want to isolate the various components:


disconnect all other USB devices
try a new cable
try a different port on the Mac
try that drive on another Mac (or another drive on your Mac)
try another power source for that drive


It's likely the voltage supply to the controller chip in the drive (or the chip itself is faulty or getting power spikes from the drive motor) so I'd consider backing up that data and contacting WD to see if they have newer firmware or if more power is needed to run that drive.