If you can get to your Desktop screen, put your Mac OS X install or restore disc into your DVD drive and wait for the disc icon to appear on the Desktop. If you double click on the disc’s icon, there is an application (usually called “Install Mac OS X”) that you can open to begin the installation process. The application asks you for your administrator password because it has to change a setting that tells your computer which disk to boot from. It tells your Mac to start up from the installation disc that you just put into the drive, rather than your hard drive.

This is what the Mac OS X Snow Leopard installation disc looks like from the Finder. You can double click “Install Mac OS X” to begin the installation process.

The restore disc has a special version of Mac OS X on it that your computer can boot from, and this is important because you cannot install Mac OS X onto the same disk from which your computer booted the operating system. Since you would normally boot your computer from your internal hard drive, your computer needs to start from another disc (your install DVD) to install Mac OS X onto your internal hard drive. When you double click the “Install Mac OS X” application from your install DVD, the computer restarts itself and begins reading your installation disc. Because computers read CDs and DVDs more slowly than hard drives, it takes much longer to start Mac OS X from the install disc.

Snow Leopard vs. Earlier Versions of Mac OS X

The Snow Leopard DVD copies much of the data it needs to install Mac OS X to your hard drive before beginning the installation. It places the data in a folder called “Mac OS X Install Data” on the root level of your hard drive. Older versions of Mac OS X, from 10.0-10.5, do not copy data to your hard drive before rebooting from the CD/DVD under any circumstances.
Booting from a Mac OS X Install Disc when You Cannot Get to Your Desktop

If you have a blank hard drive, you obviously cannot boot into your user account. The same is true if your hard drive has corrupt software on it. If you cannot get to your Desktop, there is no way to double click the “Install Mac OS X” icon from the install disc, so you need another way to boot from the install disc. Fortunately, you can accomplish this with some quick fingers.

To boot from an install DVD:

1. Turn your computer off if it is not already. If you cannot shut down your computer properly, you must hold your power button for 5 seconds. Unfortunately, you risk data loss by forcing the computer off, but if you cannot shut your machine down from the Finder, you do not have much of a choice.
2. Slide your installation disc halfway into your optical drive’s slot. You cannot push it in completely until you turn your computer on.
3. Turn your computer on, quickly finish inserting the disc into the slot, and hold down the letter “C” on the keyboard. You must do all of this well before you see the Apple logo on the screen.
4. Continue holding down the letter “C” until the Apple logo appears and the gear begins to spin. When you hold down the letter “C,” this tells the computer to bypass your hard drive and instead try to start up the computer from the CD/DVD drive.
5. If your computer ends up boot from your hard drive, then you either have inserted an incompatible install disc, or more likely, you did not do everything in step #3 quickly enough. Because you already have the disc in the drive, your second attempt should be a better one. Either restart your computer while holding down the letter “C” or do the same while turning the computer off and back on.

What if My Mac Has a Tray-Loading CD/DVD Drive?

If you have a Mac with a tray-loading drive, you need to be able to open the tray and place the disc on the tray, then close the tray. This is normally easy. If you take too long, and your machine begins to boot from your internal hard drive, do not worry. Simply finish inserting the disc and then turn your machine off. When you turn your machine on again, quickly hold down the letter “C” to begin booting from the disc.

What to Do if Your Tray Does not Open or Disc Stuck in Your Slot Drive

If you cannot open your tray (usually due to another disc being in the tray), turn your Mac on while holding down the mouse button (or trackpad button). This tells your computer to eject your disc. If you have an older, PowerPC-based Mac, you can perform this special trick in “Open Firmware” to eject the disc:

1. Turn your computer on and very quickly hold down the letters “O” and “F” along with the Command and Option keys. That’s four keys!
2. Continue holding these keys down until you see a white screen with a command prompt (it looks like a Terminal screen). Then let go of the keys.
3. At the command prompt, type “eject cd” (without the quotes but with the space) and press return. Hopefully, your CD tray opens or your slot drive ejects its disc. If a slot drive has no disc in it, nothing happens when you type this command. Unfortunately, if your drive does not open its tray or eject its disc, you may have a hardware failure and are in need of a repair.
4. Once you have your tray open or have an empty slot, insert your install disc (and close the tray if necessary). To restart your computer, type “reset-all” (without the quotes) and press return. Your Mac restarts, and you can continue the troubleshooting guide.