This article will offer solutions for those receiving errors related to the "sparsebundle" file in which Mac backups are stored.
Restart the Mac
The first thing to try is a simple restart of the Mac. Press the power button and then click Restart from the shut down menu, or go to > Restart... to initiate the restart. After it is restarted, click on the Time Machine icon in the menu bar at the top and click Back Up Now.
The Time Machine menu icon may also display an exclamation point if there was an error during the last backup attempt.
Restart the Time Capsule
If the above does not work, and you are backing up to a Time Capsule, try unplugging the power cord from the Time Capsule. Wait for at least 20 seconds before plugging it back in. When the green light is displayed, try backing up again on the Mac.
Disconnecting the network is less likely to help. Be sure to unplug the power cord, as pictured left.
NOTE: Proceed with caution if there are other users/Macs connected to the Time Capsule. Unplugging could cause data loss or corruption, especially if there is another Mac performing a backup. Be sure that unplugging the Time Capsule is not interfering with other Macs before you proceed. If your Mac is the only connected device, you do not have to worry about this.
Update your Firmware
AirPort Base Station Firmware 7.6.4 addresses the issue where you may get an "already in use" error message. Make sure you have firmware version 7.6.4 or later. 7.6.4 can be downloaded from here: http://support.apple.com/kb/DL1673. You can open AirPort Utility (built-in on Mac OS X, downloadable on Windows), select your base station, and click Update.
The above suggestions will typically solve the issue, but in some circumstances, more advanced troubleshooting is needed. Here is an excellent reference: http://pondini.org/TM/C17.html
Article ID: AT4826