Hi there macnurdin,
That is really unpleasant.
Is the drive accessible? In case it is, my suggestion would be to back the data stored on it up as soon as possible. Unfortunately, there is a chance that it is failing.
After that, you can do some basic troubleshooting as just attaching the drive with a different cable to another system.
In case the issue persists, you can test it with WD's Data Lifeguard Diagnostic tool(both short and extended tests) and see what is wrong with it: http://products.wdc.com/support/kb.ashx?id=AD9w1H
Is the drive under warranty?
If it is and it fails the tests, you can just contact the place you got it from or WD's Support and eventually RMA it: http://products.wdc.com/support/kb.ashx?id=eGLul5
Let me know how this goes,