So you’ve got yourself some Apple hardware, and maybe you’re like me. Maybe you don’t want to just run one operating system: maybe you want to run ALL the operating systems. Maybe what you want is to

Triple Boot

or even n-ary boot. The reason I use “triple” is to denote the three different classes of bootloaders in use: OSX, Windows, and GRUB. (Increasing the number of bootloaders drastically increases the complexity of the system; I recommend sticking to as few as possible.)

I also mention Apple hardware above not to evangelize or even suggest the purchase of such, but rather because running OSX on non-Apple hardware is still somewhat of a grey area. Staying cautious, I will not link instructions for that here; I will instead suggest that if one really wants OSX, you can run it in KVM.

Prep Work

First and foremost, make a backup of anything you care about. This includes ensuring you have the ability to re-install all operating systems, not just their data. (For OSX, the recovery partition is likely sufficient, but if you know how to make a bootable image I suggest doing so. I kept my 10.6 DVD handy for this.)

Along with that, you will need a 1GB+ USB stick (4GB+ is even better), a Windows install image (DVD; I’m using Win7), and a Linux install image (CD; I’m using Debian Wheezy). A wired internet connection is helpful but not obligatory, as is an OSX install image (DVD or USB stick).

Please also read the entirety of this post before making any changes to your drive(s). Since all of this information is in my head, I can’t guarantee that I mention all of it before you’ll need it. Please accept my apologies and be careful.

Partitioning

Do this as early in the process as possible because it’s the hardest to change. We will be messing with the partition table, which is a frail and brittle thing to begin with, using multiple hammers. There will be carnage and casualties, either now or down the road when Windows gets upset that it’s not the first OS on the drive (this is a thing).

Decide on a partition layout before you do anything else. Keep in mind that certain configurations (e.g., all on one drive) may require bootloaders to load each other which is incredibly delicate; you may wish to perform a proof-of-concept deployment for your partition table before committing. This time around, I have put each OS on its own drive, though I have previously put them all on the same drive (and used GRUB to load the Windows bootloader).

The best time to partition is right before OSX has been installed. When you boot the installer, after choosing a language, enter Disk Utility, and create the layout you desire. Do not create “free space” partitions in Disk Utility; instead, make them another filesystem that you will later write over (e.g., FAT32).
Barring that, with OSX booted, certain parts of the partitioning scheme can be adjusted from Disk Utility (Applications -> Utilities -> Disk Utility). This will not work for all configurations.
Boot Camp (see below) will optionally perform some minimal partitioning in the process of its operation.
Booting from the recovery partition of OSX (hold down the “option” key during boot, then select it when prompted) can provide a Disk Utility more capable than the one instide OSX.
If you have OSX install media, booting from it will provide a Disk Utility, as in the pre-install case.
As a last resort, the USB stick can be turned into a Recovery Partition of its own which can be booted to obtain Disk Utility as above. (This is useful because Disk Utility will sometimes refuse to modify the volume from which it booted.) The program to do this can be found on Apple’s website.
I know of no other ways to repartition short of reinstalling OSX.

(For the curious: yes, I did use all six of the above in roughly the order listed in order to tweak my partition configuration. This guide is very much the result of trial and error.)

Alright, go ahead and install OSX if you haven’t. Then it’s time for

Installing Windows

Put your Windows install in the CD tray and plug in your USB (doesn’t matter what’s on it; we’ll be reformatting it momentarily). Then open up Boot Camp Setup Assistant (Applications -> Utilities -> Boot Camp Setup Assistant). It will ask a couple questions; make sure to check the box to install drivers from Apple, as this is the reason for the USB.

If you are installing Windows to a different drive than OSX, when the machine powers off before booting windows, remove the drive containing OSX. This avoids Windows trashing the partition table.

Windows will restart several times during installation because installations aren’t allowed to be simple. When it finishes, open up the USB, go to the “BootCamp” folder, and run the “setup” application. This will take a few minutes and install any drivers needed for your system and a bunch of others you don’t need. Do not be alarmed if Windows does not understand your display or graphics card during the installer; the drivers will be installed when the application finishes. It will require a restart.

At this point, switching between OSs will require holding down the ‘Option’ (‘Alt’) key during boot to use Apple’s boot menu.

Installing Linux

This is oddly enough the trickiest part of the installation. Apple systems boot using EFI, but it’s not the EFI you’ll find on any other system. They switched to EFI booting between two versions of the specification, so it has features of both. The result is that Apple EFI is effectively a different boot method in that if the boot process is not coded for it explicitly, it will likely not work.

The result of the above is that I will not be creating an EFI-booting Linux. If you wish to do so - and there are reasons, mostly hardware-related, why you might want this - you will unfortunately have to look elsewhere; I’m happy just having this working at all.

The key factor to booting non-EFI Linux (using BIOS emulation) is to have an MBR present on the drive that Linux is booted from. If Linux is the only OS on the drive, then it’s conceptually easier: just use an MBR on the drive. If it’s also on a drive with an EFI-booting OS (OSX, Windows, etc.), then with some trickery one can place an MBR onto a drive that uses a GPT.

If you’re going the Linux-only drive route, then you need the drive to have an MBR on it before you install Linux. To do this with a Debian install image, just use an Expert install (a regular install will not work, since d-i defaults to GPT on Apple drives). If you’re not comfortable with that, or using a different image, you can use fdisk instead. You don’t need to set up the partitions themselves here; just ensure that it has an MBR. Then install your Linux as normal.

Otherwise, install Linux without overwriting the partition table. This is why it’s important to partition everything out beforehand, allowing space for the Linux volume on the OSX drive, for instance. Then you need to sync the GPT with the MBR that Linux has created. The easiest way to do this uses a tool called rEFIt. rEFIt’s website has a notice that it’s no longer maintained (which is true) and recommends rEFInd instead (I disagree with this). rEFInd will not sync the GPT and MBR in the way we need, so it’s not useful. Another way is to use the gptsync utility.

Finally, even if you’re not installing Linux on the same drive, you may wish to install rEFIt as a boot manager anyway. It looks like the rEFIt installation process may be borked under OSX 10.10, which is unfortunate; I’ve heard tell that manually installing it will work, but I did not test it. rEFInd may be capable of performing this task, but again, I have not tested.

My Setup

I have a MacPro4,1. The first drive is 80GB of OSX 10.10 only; the second drive is 600 GB of Windows7 only; and the third drive is 600 GB of Debian Jessie only. I’ve swapped the graphics card for an ATI Radeon HD 5770 (read: this works without EFI booting). The wireless under Linux require the proprietary driver (b43 does not support N on this card). The only quirk I’ve uncovered is that going directly from Linux to Windows without going through OSX first results in the wireless not working, which can be fixed by booting OSX and then the desired system.

Previously, I owned a MacBook3,1. At the time I was unaware of how to get Linux-only setups, so the first 20GB of its drive was an unused OSX 10.6, and the remainder was Debian (initially Lenny, then later Squeeze and Wheezy). b43 did not yet support the wireless card, so it was using wl, though the graphics worked fine. I did have it booting Windows as well, though this introduced too much instability (Windows ate the partition table at one point) and was quickly removed.

Acknowledgments

I did not write any of the tools mentioned in this post. Much of this is the result of my own trial and error, though I have also gathered information from the Arch wiki and the Debian wiki (to which I have added back relevant portions of my findings).