I had the same problem trying to repair a disk on a WD external hard drive.
I googled for the answer, read several forums, and finally called live tech support at WD. None of this was helpful, but added up to a big waste of time and growing frustration.
For me, I stumbled on the simple "duh" simple fix: Force eject all the disks the volume (to do that I had to first go to my Activity Monitor and quit any programs that might possibly be using any of the disks in any way); then shut down the computer & turn in back on; And then re-connect the external drive back with the computer. After doing this, I ran disk utility and it said the volume and all its drives appear to be okay.
Good luck.