I have experienced the same problem on a MB Pro 2011 and a 2013 MBAir both with updated OS's. OSX10.9.3. WD MYBookLIVE 3Tbyte. I have found the only solution is to reboot the WD device thru Western Digital's software that you can access via the IP address for the WD device. For example, if you type the WD backup device's IP, like ... "125.261.1.38 " in the Safari address bar, WD has built-in software the runs a program that allows you access to the WD unit to maintain it, turn it off, reboot it, partition it, etc.
Doing this, I believe disconnects or ?dismounts? the WD from TM and resets everything. It works most times, not always, Sometimes you need to reboot the Mac too.
I'm more interested in what is causing this frequent failure on the WD. It happens to me several times per week. I suspect, network failures or improper disconnections between WD and the Mac (not caused but the user) are the culprit but I haven't seen any blogs on a permanent fix. Spent hours on the phone with WD techs; they were clueless. I found the "fix" myself.
Further, I often get an associated reply to "dismount the WD" with the "sparse bundle.XXX is in use" error. But when I go to Finder or Disk Management to find out what's mounted on the Mac, the WD is never mounted, even when the TM backup is working properly. I find the WD device under SHARED in Finder. Its never mounted, so I can't dismount it.