There doesn't seem to be a single cause for this message, so there doesn't appear to be a single fix, either.

Things that have helped in some circumstances:

1. •Be sure no other users or Macs have it mounted.  If so, have them select it on the desktop or Finder sidebar, and eject it manually.  (That sometimes happens if it's still being indexed after a prior backup, or the backups were being viewed via the "Star Wars" display). 
   

2. •Disconnect all users (per item C12), then go to Time Machine Preferences and re-select your Time Capsule or network location as the backup destination.
   

3. •Restart your Mac.
   

4. •If you've changed the network volume's password, see item C14.
   

5. •It's possible some names (of all things!) may be a problem.  See item C9.
   

6. •The backups may be corrupted.  Try repairing them, per item A5.
   

7. •If you have WD SmartWare installed on Lion 10.7.x or later, it may not be compatible, and there are reports it can cause this problem as well.   See if there's a newer version; if not, use Western Digital's uninstaller, or delete the app from /Applications and the files com.wdc.WDDMservice.plist and com.wdc.WDSmartWareServer.plist from /Library/LaunchDaemons.   There may also be a file in Library/Application Support.
   

8. •One user reports having very little space on the startup drive (4 GB), and freeing more helped (a general rule is, try to keep at least 10% free on your startup volume).

11/15/12

Copyright © 2012 James Pond.  All rights reserved.