Sometimes you will want to start up your Mac from a volume (hard disk, CD/DVD, pen drive etc.) which is different to the one you usually use.  The Mac is configured to start from the internal hard disk by default, so how do you tell it to start from somewhere else?

startup-diskThere are a couple of ways to do it.  If your Mac is starting normally, you can simply go to the Apple Menu and select System Preferences.  Then, look for the Startup Disk panel.  This panel will show you all the disks that you have available to startup from.  Select the disk you want, then click the Restart button.  The Mac will restart and it will attempt to boot up from the disk you've selected.

If your Mac is not able to start normally and you therefore can't get to the Startup Disk panel, you can use the other method.  First, switch off the Mac.  Then, make sure you have connected the disk you want to start up from.  (If it's a DVD, you will have to quickly slip the DVD into the drive immediately after pressing the power button.)

bootNext, you need to hold down the Option Key on the keyboard.  Sometimes it's labelled as Alt.  Keep holding down this key while you switch on the Mac and keep holding it until you see the Boot Volume Selection Screen.  This screen will show you all the available volumes that you can use to boot, much like the Startup Disk panel.  Use the arrow keys on your keyboard to select which volume to boot from, then press the Enter key or click the arrow to start booting!

This is the same procedure that you should use if you need to boot from the hidden Recovery Partition on your Mac.  Booting from the Recovery Partition will allow you to run tests on your disk and perform other disk related actions, as well as providing the facility to completely re-install your copy of Mac OS X if that's what you need to do.