Can't Install Mountain Lion - Disc doesn't use GUID. How will i make it work?

Question

Hey.
I just bought and downloaded Mountain Lion,
but when i try to install it on my Macintosh HD, it says that the Disc doesn't use GUID.
How will i make it work?

Answer 1

I'll explain a bit what is going on so you will understand.
Storage and boot drives have instructions at the begining that tell firmware and hardware what partitions are on the drive and their format, among other things.
This is called a Partition Table or Map, and there are different types. One is GUID (GPT) that Mac's and OS X uses now to boot from and Master Boot Record (MBR) that is used with older Windows XP PC's and files drives today come with.
Somehow your Partition Table got changed from GUID to MBR, how this occured is unknown but you have been able to use the computer and OS X until today.
To change the Partition Table requires all data on the drive to be removed as basically the map is going to be changed so the parittions (even if one) on the drive is going to lost it's "map".
Despite what Disk Utility tells you, you have two partitions on the 10.6 boot drive, a hidden EFI partition also exists. (and in 10.7/10.8 there is yet another called Recovery)
So EFI is going bye bye along with the OS X partition, however you can clone the OS X partition to another drive either using Kappy's method, or Carbon Copy Cloner or SuperDuper.
So you first need to format a powered, blank extenral drive using Disk Uility (select the drive makers name and disk size on teh far left) with a Partition > Options GUID and Format OS X Extended Journaled, quit, then use CCC to clone the OS X Macintosh partition to the extenral drive.
Once this is completed you can hold the option key down to boot from this external drive and check it out. You also should make a drag and drop copy of your files to yet another drive as a extra safety net. Disconnect this last drive.
Now what you want to do is while booted from the clone is use Disk Uility to format the INTERNAL drive the same way you did the clone drive, with a GUID and OS X Extended Journaled.
Now use CCC to clone the external drive to the internal drive, while option key booted from the external drive. When that's finished you need to change the name of the external drive to something else preferablly.
When your done, disconnect the drives and reboot, set the Startup Disk in System Preferences to the internal drive.
If you get a flashing question mark upon boot, hold the option key down and select the drive to boot from then repeat the above step.
Now you should have a GUID partition table and ready to upgrade to 10.8.