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Installing Mac OS X onto a disk having the MBR Partition Map Scheme

If you try to install OS X for Intel CPUs from DVD to a disk that is not formated as GUID Partition Table (GPT), the installer won't let you. The reason being that an Intel Mac is not prepared to boot off any other partition schemes (it needs an EFI System Partition, which can only be installed on a GPT scheme) - but a PC can boot off a MBR-partitioned disk, so this imposes an unnecessary restriction.

One work-around is to patch the OS X installer, but that requires a running Mac OS system and also a disk into which you need to copy the DVD contents.

I have devised a simpler method:

Modify the Partition Scheme in place from MBR to GPT, then install OS X from DVD, then revert the change to be on the safe side.

The modification is not too hard, provided that the current disk partitioning only uses up to four primary partitions and that there is room between the MBR and the first partition. On the Asus Eee PC 1008HA, for which I've worked out this solution, this is the case. The process involves creating a new GPT, writing it to the blocks 1 to 33 as well as to the end of the disk, and turning the first block into a "protective MBR".

This does not create a truly valid GPT, because not all flags and partition types are correctly translated, and the disk's boot loader will get disabled, so that one can not boot from the disk any more (a separate boot disk with Chameleon is required anyways to install OS X, so this should not be an issue at this point). However, it is enough to install OS X. Afterwards, the partition modification should be undone so that the disk becomes properly bootable again, and also avoids any other possible side effects.

I am currently writing a program that can perform this modification (after first checking that the requirements are met). It also writes a journal file that can be used to later undo the changes again. It works on Mac OS X, Windows and even Linux, if someone should desire so.


Unfortunately, I just had to learn that the above is a bit more complicated after all! While my GPT gets detected as desired, OS X still tries to add a new EFI partition, and fails at that for odd reasons, such as if the previous partition started on an odd block number (which is the case with the Eee PC I'm working with). So it appears that I have to also add these partitions myself in order to work around that bug. This will take some more time. Soon.


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Page last modified on 2009-07-01, 21:02 UTC (do)
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