dual-bootmacosunix
I have a Lion installation dual-booted with Ubuntu using refit. Upon trying to upgrade to Mountain Lion, I reboot into the mountain lion installer, and get a message about OS X not being able to be installed on this disk since it can't be booted.
How can I recover from this situation? My machine is not having a problem being booted (I can work around that).
I ended up figuring it out the restart issue still unresolved but doesn't really matter at this point as the second part I resolved on my own by doing a PRAM/NVRAM reset (command-option-p-r) at startup after this it installed fine I wish I had thought of that before wiping my Mac OS X partition.
take a full backup to an external usb drive using a tool like Superduper or CarbonCopyCloner and boot it to make sure it's good (select disk using "Startup Disk" in System Preferences.) You need a good backup before messing with the partitions.
Using Disk Utility (/Applications/Utilities/Disk Utility.app) select your internal drive in the left-hand column and select the "Partition" tab in the right-hand column. Hit the "+" button under partition layout to create an additional partition. You can use the slider in the partition layout or the "Size" textbox to change the partition sizes. Hit "Apply" to confirm changes and write to disk.
All partitions will be dynamically resized, presuming they pass a fsck (filesystem check test.)
That's all there is to it! I've done this on an iMac and a Macbook Pro so far this week with minimal problems. Only the Macbook Pro couldn't pass the fsck and I needed to boot to the recovery partition to repair the root fs before I was able to shrink and create another partition.
Addendum: repairing the system partition
The system partition cannot be resized if it does not pass a filesystem check and you cannot repair the system partition while you are booted into that system so you will need to boot from an alternate one. Apple has made this very easy by automatically creating a Recovery Partition in OSX Lion and later.
A lot of text will scroll by in the text box. If all goes well you should end with a message telling you that "Volume repair is complete."
Best Answer
I also had this problem (on a 2012 MBA). Running rEFIt's partition table tool and accepting the "update the partition table" dialog it offered fixed it for me. (The Ubuntu install instructions at https://help.ubuntu.com/community/MacBookAir4-2 do tell you to do this to resync the partition tables; but not doing it didn't seem to cause a problem at the time.) For me the only partition table change it made was to change the type of the recovery partition to Apple Boot: probably best to look carefully at what it is proposing on your system before accepting it.