Mac OS X
If you have a hard drive over 8 GB in size, you must partition it or you will not be able to install Mac OS X. If you are creating the partition within OS X, it must be smaller than 7.4 GB as reported by Disk Utility (because sometimes a GB is billion bytes and sometimes it's 1,073,741,824 bytes); we suggest simply setting it at 7 GB to avoid having to redo the whole installation if the partition ends up bigger than specified (it happens). Mac OS X must be completely within the first 8 GB of space on your hard drive or you will not be able to run OS X.
Tray-loading iMacs cannot boot from USB drives (see Apple Knowledge Base Article #58430, USB Info and Benefits of Dual-Channel USB).
Non-Apple upgrades and peripherals (such as unsupported USB devices, replacement drives, and third-party memory) may cause problems when installing or booting into Mac OS X.
Be sure that your iMac's firmware is up to date before you install Mac OS X, and read and follow all of Apple's "Read Before You Install" instructions to increase the likelihood of getting OS X installed and running on the first try.
