AirPort Time Capsule: Initial backup is interrupted
Your first Time Machine backup will be several gigabytes because it includes all files on your computer. It could take overnight, or longer, depending on how much data you have. Once the initial backup is complete, all subsequent backups are incremental.
It is recommended that you do not interrupt the first backup.
If the first back up is interrupted, when you resume, Time Machine verifies the integrity of the first partial backup. This takes some time and during this period the backup appears to progress very slowly. This is normal and your backup will resume once the integrity check is complete.
If you have an Ethernet cable, you can perform your first backup to your AirPort Time Capsule via Ethernet; this will allow for the fastest initial backup especially with client computers that use 802.11b or 802.11g.
If an interruption occurs, Time Machine will wait for a period of time to see if the AirPort Time Capsule becomes available again. After this time expires you will see a alert message stating "Time Machine Error. Unable to complete backup. An error occurred while copying files to the backup volume." In Time Machine preferences, the "Latest Backup:" field will state "Failed".
If this alert appears: "The storage location for Time Machine backups can't be found," open Time Machine preferences or click its icon in the Dock. Reselect your backup storage location.