Why won't Mountain Lion run on some MacBook Pros?

Answer

bcastle
Vote Up (22)

I agree.  Some of the early 64-bit Macs sometimes have 32-bit graphics drivers and that is causing the compatibility issue.  If you can figure out whether you have a 32-bit or 64-bit EFI, you should be able to tell whether you are good to go or not.  This is the command to run in Terminal that will tell you - if it returns "EFI32" you will have a problem running Mountain Lion:  ioreg -l -p IODeviceTree | grep firmware-abi

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