Contributed by: tofergregg
If your bootloader gets changed or corrupted, this fix will restore it without damage to your disk partitions.
This hint may be a bit esoteric, but I thought I was up for a long night of reinstallation pain before stumbling upon this fix. I made the mistake of trying to use an Ubuntu 9.04 boot CD to install Ubuntu to an external (USB) drive on my Mac.
Don't do this, unless you know the following: Regardless of the fact that you chose the external drive upon which to install Ubuntu, you won't be able to boot back into your Mac without changing the bootloader. I ended up with the dreaded question mark folder when I tried to reboot my Mac, and nothing worked to boot into my OS X partition.
Luckily, I have a bootable external drive with OS X on it, and I was able to boot into it by holding down the Option key (the primary partition still did not show up).
I started searching online for the fix, and it seems that Ubuntu changes the Darwin (default) bootloader to Grub on your internal disk, and OS X won't use it (at least not by default). The fixes listed were kludgy, and amounted to either installing another third party bootloader (called "rEFIt") or completely wiping your internal drive and reinstalling OS X (even a standard install won't work).
Anyway, the fix turned out to be easy with Disk Utility (and probably possible from the OS X boot disc). Run Disk Utility and click on your internal hard disk (the disk itself, not the partition under it). Then click on the Partition tab. If you move the triangular slider that adjusts the partition up and then back to where it was, the "Apply" button becomes active (it starts greyed out). You can now click "Apply," and the partition will be left alone, but the bootloader will be recreated.
When I did this, a window popped up that said, "Are you sure you want to partition the disk? Partitioning this disk will change some of the partitions. No partitions will be erased." If you get a message that partitions will be erased, I'd look into it more before going ahead!
I was surprised that this worked, but when I rebooted, the computer booted straight into OS X from the internal drive.
http://forum.ixbt.com/topic.cgi?id=79:4091
