My Time Machine backup has stopped working and I don’t know why. I see a message about a ‘sparse bundle’ being in use. How do I fix my problem?
This is most often associated with a confused AirPort Base Station or Time Capsule (more often with a Time Capsule that’s used to back up more than one Mac). The first thing I’d do is launch AirPort Utility (found in /Applications/Utilities), select your base station or Time Capsule in the resulting window, and click on it. In the small window that pops up, take a peek at the version number. If it’s not 7.6.4 you should update it. Among the changes wrought by this update is: “Resolve(s) an issue that may cause AirPort Time Capsule to report that the backup disk is in use.”
To update your device just select it. If an update is available an Update button will appear in the resulting window. Click it to update the device.
If your base station or Time Capsule is already up-to-date and you’re having this problem, restart it. You do this by selecting it in AirPort Utility and choosing Base Station > Restart.
Still having problems? Try rebooting your entire network. Do this by depriving your network devices of power – the broadband modem as well as your Airport device. Then bring them back to life, powering up your modem first and waiting for it to make its internet connection. Once it has made that connection, plug in your base station or Time Capsule, wait for its lights to turn green, and then see how Time Machine behaves. You should no longer see the error.
by Christopher Breen, Macworld