OS X Mountain Lion: If a message says the identity of the backup disk has changed
As a security measure, Time Machine won’t do a backup if it detects that the identity of your backup disk has changed. This can happen for several reasons:
You erased or reformatted your backup disk or partition.
You deleted the disk image for your backup disk.
You attached another disk to your computer with the same name as your backup disk.
Someone is trying to steal your information by tricking your computer into backing up to an unauthorized disk.
If you attach another disk to your computer, be sure to give it a different name from your backup disk. Then make sure your backup disk is correctly identified by opening the Time Machine pane of System Preferences and re-selecting your backup disk.