How to boot off an external hard drive:

• If netboot is not working for some reason, it can be useful to boot off an external hard drive to run tests on the computer.
• If the machine is on, turn it off.
• Connect an external hard drive to the firewire port of the computer.
• Hit the power button and hold down the option key.
• A screen will appear with some buttons in the middle of the screen, a button with a straight arrow on the right bottom, and a button with a arrow in a circle on the left bottom.
• The buttons in the middle of the screen are all the different ways that you can boot the computer.
• Choose the external hard drive by clicking the appropriate button.
• Then boot the computer with the external hard drive by clicking the straight arrow on the bottom right of the screen.
• The arrow in a circle is used to refresh the boot options in the middle of the screen.