I have been in contact with support. They think that because my drive wasconnected during my Yosemite upgrade, it corrupted something. I find that hard to believe but it's plausible.
I have two Western Digital My Book Studio II (RAID 0) 4GB external drives and both were on version 1015 of the firmware. One was connected via Firewire and powered on when I upgraded to Yosemite and the other was powered off. The drive that was powered on during the upgrade will not mount in OS X 10.10 or on another computer I have running OS X 10.9. But the drive that was off during the upgrade will mount on 10.10 or 10.9 just fine.
I first tried upgrading the firmware using the WD Universal Firmware Updater on my OS X 10.10 computer but there is a Java conflict that I couldn't get past. So I used my OS X 10.9 computer to update the firmware on both drives from version 1015 to 1017 which did not help with the mounting issue but maybe that will help someone else with this problem.
The engineer from WD had me check to see if the unmountable drive would appear in Disk Utilities and it did. However, the partition under the drive was greyed out and when I tried to Verify it, it failed. I tried this on both OS X 10.10 and 10.9 with the same fail result. The engineer then explained that it's likely that the Yosemite upgrade did something to the drive.
After a few other troubleshooting steps, the engineer consulted with other engineers and came back with a free software license for "Easy Recovery 11" which after hours of scanning was able to locate all of the files on the drive and restore them to another drive. Once recovered, formatting the drive made it possible to mount again.
Clearly something happened to the partition on the drive during the upgrade that caused it to no longer mount. This could very well be a problem with the Apple upgrade and not Western Digital's fault. I will not be leaving drives connected during future upgrades, that's for sure! First time in 20+ years I've had an issue quite like this.
If your situation is like mine where the drive is viewable in Disk Utility but the partition fails on Verify, you probably need to give up on drivers or a software update fixing this and make plans to recover your data and reformat the drive. I suggest opening a ticket and then calling them at 1(800) 275-4932 (I think it was option 4).
Best of luck!