Want to install Linux on an iMac? This HowTo should help you with the most critical points. This HowTo works for many different Linux distributions, including Ubuntu, Linux Mint, Fedora etc.
1. Keep Your Mac OS X for BIOS Updates!
Not just because it's nice to use from time to time but also because of the IMac's software BIOS you really need it to profit from bios updates, here is why: On a standard PC the BIOS (the most basic piece of software that makes your motherboard work) is stored in a little chip on the mainboard itself. When there is a BIOS update, the software on this very chip (I guess the correct term is flash memory) gets updated. I suppose there is also something like this on the Imac Motherboard, but at least the updated BIOS is loaded from the hard disk. The real advantage of this is that you don't risk the fact, that your machine becomes unusable, after something goes wrong during an update. The disadvantage is that you need to keep Mac OS X installed, if you want other operating systems to make use of BIOS updates, and sometimes these updates a really usefull. The latest update e.g. fixed the problem, that you weren't able to use the USB keyboard in a Linux Bootloader Menu (I think because of lacking USB Legacy Support). This understood the actual OS-Installations and Troubleshooting can be approached.
2. Installing Mac OS X and Linux
So you want to install Linux next to MAC OS X. The normal scenario would be, that there is already a OS X, that pretty much uses all of your hard-drive space and that needs to make some room for Linux. There are definitely ways to shrink the OS X Partition with Mac OS X or even with Linux, but I'd prefer a nice and clean new installation of OS X and Linux, but that's my choice. How ever you decide, keep in mind, to not only make room for Linux itself but also for a linux swap-space. Also keep in mind that Mac OS X is not free and you need to buy it and Linux is free. For installing Mac OS X get it and install it referring to the manual. For Downloading and Installing Linux, please refer to this page.
This HowTo supposes at this point, that your Mac OS X and Linux installations went fine but after powering it up, your Mac automatically loads OS X with no possibility to choose between OS X or Linux at startup. That's why first of all you need a bootloader to get this possibility.
3. Installing the Bootloader
One would think, that since Linux comes with a bootloader, you don't really need something else, but because of the Software-Bios design described above, you need a bootloader that can deal with it, I'd describe it as a Mac OS X compatible bootloader ;)
The nicest I found is the result of a very nice open source project called rEFIt. The version I used (0.14), gives the option for a better automatic selection on bootup, since you can now configure it to either automatically choose OS X or Linux or Windows.
You can rEFIt and find all the documentation about it from this page. Please notice that you'll have to reboot twice, to actually use the boot menu, when it still doesn't come up on boot, try holding the "Option"-Key (="Alt"-Key) when switching your IMac on. When it still doesn't come up, check out the rEFIt Documentation. But don't panic, normally the menu comes up automatically, after you brought it up by holding the "Option" Key it definitely comes up everytime you switch on your machine. The only time you might to have to deal with holding the key on startup again could be after a Mac OS X update.
To configure which OS is loaded automatically on bootup, and how long you have time to choose a different one, is actually pretty easy, you only need to edit rEFIt's config file, which is commented so good, that you can't really do anything wrong.
The only things you need to change here is the line "timeout". E.g. if you want to let default selection beeing booted in 2 seconds, the line would be
For changing the default selection jump to the end of the file, where you should find the "default_selection" line. As described above this line, the options are L for Linux, M for Mac OS and W for Windows. So for choosing Linux as default, the line would be
At this point you are allready able to use Mac OS X and Linux, choose between them on startup but also have configured a automatic selection that is booted, when you don't do anything during startup. The following will fix some annoyances, you probably only notice after a while using it.