Please be aware that there are serious data loss issues ongoing with Mac OSX Mavericks and Western Digital drive sofware. Both the Apple support and WD forums have multiple customer reports of total external drive data loss.
The data loss is not limited to systems activly using the WD software and drives, other vendor external hard drives are being lost.
Personally I just lost 1TB of Time machine backups on a Iomega external drive. I did have a WD myBook for a while about 2 years ago but was not aware the WD drive software was still active on this machine.
Current advice from Western Digital is to uninstall the WD drive software before going to Mavericks. Uninstaller is here
This situation as already caused a lot of digital pain, loss of data and as a technical person makes me sad in so many ways.
Only solution Erase, Repartition and make a new volume. The disk was stuck for in a not mounted state and not able to be erased saying disk could not be "Un mounted." Fix for that was to find the running fsck_hfs process and kill it. Pointed TimeMachine at the new volume and replaced the Old volume name.
To the data distruction digital hall of shame we add ....... Western Digital
and for lack of governance and oversight of vendors ... Apple