The previous backup may be corrupted, or the folder where backups are stored may not have suffiicient privileges. First try deleting the current backup. In iTunes, Preferences, Devices tab you can delete a backup. If you have Windows sometimes this does not completely delete the backup, so go to its location on the file system and make sure it is gone. If it isn't delete the whole backup folder.
- Mac: ~/Library/Application Support/MobileSync/Backup/
- Windows XP: \Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\
Note: To quickly access the Application Data folder, click Start, and choose Run. Type %appdata% and click OK.
- Windows Vista and Windows 7: \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\
Note: To quickly access the AppData folder, click Start. In the search bar, type %appdata% and press the Return key.