The first of maybe 3 or 4 videos showing you how to triple-boot your Mac to make it run Snow Leopard, Windows 7, and Ubuntu.
** ALWAYS MAKE BACKUPS OF YOUR FILES BEFOREHAND! **
The first part of the series tells you what you need to do in Mac OS X:
1) Install rEFIt - Install rEFIt here: http://refit.sourceforge.net/ Install it, but don't restart if it asks you to (well, you can, but you don't really need to). Just minimize the installer.
2) Partition your drive with Disk Utility - While this doesn't remove anything, it's always best to have a backup (Time Machine works very well). Add 2 new partitions to your drive, and size them out however you like. Give them good labels. Format the both as FAT. Please keep in mind that the partition in the middle (the second one) will have to be your Ubuntu one. Also remember the size of these partitions, as this will come in handy when installing the other two operating systems.
The next video will show you how to install Windows 7 on your Mac. If you'd like, you can do this now. Just put in the DVD, hold ALT/Option when you turn on your computer, and choose the Windows disc (not the EFI boot one). Choose "Custom/Advanced" when you're in the installer, and click on your Windows partition (that you made in Disk Utility). Click on "Format". This formats the partition to NTFS. You can then begin the installation.
61