This is especially true of USB flash drives. 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. Use the keyboard arrows to choose your boot device, then press the Enter key. Continuing to hold this button down will bring up a menu where you can select a disc or drive to boot from. 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. These are all potential reasons for booting to an external device, among many others. 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. 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. 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.

Apple made it easy so all you need to know is just a simple keyboard command.

At some point, you may find a need to boot your Mac from a disc or a drive other than the primary Mac OS X startup volume.

External mac dvd drive will not load