System Preferences - Startup Disk

The Startup Disk preference enables you to select a startup disk and if desired, restart your computer. This is helpful if you need to start your computer under Mac OS 9.2 or from a bootable CD-ROM disc.

Mac OS X allows you to have multiple System folders on a single disk or partition. Startup Disk is the tool you use to select which System folder is required at startup.

To select a startup disk
Click on the icon that has the startup folder or disk you want to use.

Click the restart button or quit system preferences and click the change button in the confirmation window that appears to save your changes.