Restore a Damaged GUID Partition Table Startup Disk on OS 10.6
If you’ve ever stuck the live CD of your favorite Linux flavor into your MacBook Pro, you’ve probably also booted to a lovely flashing folder icon afterwards and panicked. Fret not, the solution is simple!
Hold down the option key during boot. Select the Startup volume (the one farthest left) and boot. Go to System Preferences, Select Startup Disk. Select the hard disk farthest to the left again, or the disk which you wish to boot to, then select Restart.
You’ve just repaired the Header of the GPT and fixed your problem!