Dual Boot Apple

Purpose

To dual boot on an Apple machine, for instance between OS X and FreeBSD.

Details

The default EFI boot manager on Apple machines is tuned for working best with OS X installations. What a surprise. In order to take advantage of the overpriced hardware at your disposal, yet from another operating system (let it be a GNU/Linux distro, one of the *BSD, or even Microsoft Windows), it is necessary to modify the partition table of your drive, as well as installing a more powerful boot manager. Once this is done, the installation process of one or several other operating system will vary depending the selected OS. Eventually you will be able to choose at boot time which OS you want to load.

Step by Step Recipe

• We start assuming that your OS X installation is one single partition on an internal or external drive;
• Install rEFInd boot manager
• Start Disk Utility and resize the OS X partition to give enough room for the extra OS (for the record Mavericks needs at least 8GB, so reducing the OS X partition to 20GB should be enough for a basic install and a couple of closed source bloatware, however the Disk Utility may prevent you to limit the space that much depending on the drive specs and your current configuration);

Debian Installation (GPT and EFI compatible)

• Get an (U)EFI compatible Debian netinst USB or ISO (for instance http://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/daily-builds )
• Boot into the Apple EFI boot manager holding the alt button
• Select EFI boot for the CD or USB (If there is no mention of EFI for the installer, you probably have a non-EFI installer, get another one)
• Proceed with the installation
• At the partition screen, select guided partitioning, using the largest continuous free space and select a partitioning scheme of your liking (to simply things here I assume "All files in one partition" has been selected)
• The partition table should look like this:

    FREE SPACE
#1  ESP          EFI System
#2  hfs+         Your OS X install
#3  hfs+         Recovery HD
    FREE SPACE
#4  ext4         Debian
#5  swap
    FREE SPACE

Note: Here I edited #4 to name it Debian.

• Finish partitioning and write changes to disk
• Finish installation and reboot
• At this point of the installation this what you will get:
  • default boot ends up with GRUB pointing to two non working Mac OS X entries
  • EFI manager boot (holding alt) ends up with the regular OS X startup

Troubleshooting

If you can't get back into OS X because you blessed the wrong partition or you managed to make a mess of GPT, you can always try the following: boot the machine, after the chime press the alt until you see a list of available OS X partitions that can be booted.