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.
