Let’s say you need to use the Mac OS X installation disc that came with your computer to reformat the hard drive and put it back to factory settings. Or maybe you’re trying to boot from a USB flash drive that has a clean install of OS X on it for troubleshooting purposes. Perhaps you’ve got a cloned backup of your entire Mac on an external hard drive and you want to make sure it’s bootable. These are all potential reasons for booting to an external device, among many others.
There easiest way to boot to any device other than a Mac’s internal hard drive is to press and hold the Option key immediately after hearing the Mac startup chime. Continuing to hold this button down will bring up a menu where you can select a disc or drive to boot from. Use the keyboard arrows to choose your boot device, then press the Enter key. The computer will start up from the chosen volume, but bear in mind performance will likely be much slower than when you normally operate your Mac. This is especially true of USB flash drives.
Rather than hold the Option key, you could instead just press & hold the C key if you’re booting from a CD or DVD disc. This will bypass the selection menu and immediately start from the disc. It won’t work for USB and FireWire drives, though.
Booting to another volume using either of these methods is a one-time temporary change, so you don’t have to worry about altering any settings to reverse it. Your Mac will go back to booting to it’s primary startup disk next time you reboot.