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We installed Ubuntu 12.04 and updated to 12.10 (Raring Ringtail) on a separate partition of our Mac tower. However, when trying to load Mac OS X from Grub, we get an immediate kernel error along the lines of "invalid kernel," which states that no Mac OS version has yet been set. Ubuntu itself works flawlessly, and we can see that all the data on the Mac OS partition is perfectly fine. What is the likely issue, and how can it be fixed?