As the OS updates it can cause another issue for WD to find and correct. Is an Apple Time Capsule an option?
I have two MacBook Pros and keep the Operating system current. I also have a Western Digital 2T My Book for backup on each computer. Regularly I receive error msgs saying, "Time Machine was unable to complete backup" My WD firmware is also current. I don't understand what is occurring? This occurs on both Macs. WD tech support has helped a couple of times, but the "fix" doesn't stick.