A bug apparently exists in Apple's Time Machine backup routines in Mountain Lion that can prevent recent backups from being used for a full restore.
By default, Time Machine mirrors the entire boot drive of an OS X system, and should update on an hourly basis to keep the backups current with changes to your system. By combining hard links to old backups along with new backup data, it quickly creates a full representation of your system by copying only changed files to the backup.
In this sense, in its default configuration each backup represented in Time Machine should be a fully restorable backup instance; however, recently users are finding that in OS X Mountain Lion this is not always the case.
Apparently some corruption is occurring in the Time Machine preferences file that is preventing more-recent backups from appearing as valid backups for use during restoration. This problem not only prevents the backups from being listed as valid restore points, but also keeps migration tools like Apple's Setup Assistant and Migration Assistant from using them.
To check whether your backups are experiencing these problems, first go to the Time Machine menu extra (enabled in the Time Machine system preferences) and choose "Back Up Now" to manually create a new backup instance.
When the backup is completed, reboot your system to the Recovery drive by holding the Command-R keys immediately after hearing the boot chimes at startup. At the OS X Tools window, choose the option to restore from backup, and then see what backup instances are available. The latest backup you created should be listed as the first restore point available.
If the available backups listed do not include this latest one, and especially if the only ones available are from a while ago even if you have had Time Machine running regularly, then it is likely you are experiencing this bug.
Unfortunately, for now the only known way to fix this is to reset Time Machine and start over, which can be done by the following steps:
- Turn off Time Machine in the Time Machine system preferences
- Delete the file called "com.apple.TimeMachine.plist" in the Macintosh HD > Library > Preferences folder
- Start Time Machine in the System Preferences
- Add your drive as a Time Machine backup destination again
When you do this, new backups created on the drive should now be accessible. Prior backups on the drive should continue to be available.