I bought a My Book for Mac 2 TB drive to be used with my iMac. I run OS X Yosemite. After trying unsuccesfully to get it to work with Time Machine, I contacted Western Digital and was told:
“This drive is not compatible with Yosemite, it may be why you were not able to use it with Time Machine. We extend our apologies for the inconvenience. In regards of your case, we do not have information on a release date for an upgraded firmware or driver for the Yosemite compatibility. You may wish to join, and add to our Community Forums’ Ideas Lab at the link below. Our development team reviews these posts on a regular basis, to see the most commonly requested features by our customers.”
So there it is folks. When is a fix coming? Seems to me to be a pretty basic requirement to have an external drive named My Book for Mac at least be compatible with Mac operating systems.