Mac OS X 10.6: Changing your startup disk
You can make your computer start up from a CD, a network volume, a different disk, or another operating system. To do so, you change your startup disk. You can change your startup disk only once—the next time you start up the computer—or you can set your computer to always start up from a different disk.
To change your startup disk once:
Hold down the Option key as you restart the computer. When you see the available startup disks, select one.
If your computer is on a network and a network startup disk is available, hold down the N key as you restart the computer to start up from the network startup disk.
The next time you restart your computer, it will go back to starting up from the disk selected as your permanent startup disk in System Preferences.
To change your startup disk:
Open System Preferences and click Startup Disk.
If necessary, click the lock icon and type the name and password for an administrator user.
Click the icon of the disk you want to use, and then click Restart.
Warning:When selecting a network startup volume, make sure you select a valid network startup volume and not a network install image. Choosing a network install image reinstalls your system software and may erase the contents of your disk. A standard network volume icon appears as a globe and Mac OS X system folder. When you select a network volume icon, a message appears in the Startup pane describing the volume. A network install icon appears as a globe with downward-pointing green arrow.
If you don’t see a disk that you expected to see, that disk may be able to start up some computers but not your computer. For example, a disk that can start up a PowerPC-based Mac may not be able to start up an Intel-based Mac.