Startup Disk control panel

The Startup Disk control panel appeared as of System 4.2 (then called Startup Device) and could be accessed from the Control Panel desk accessory. Starting in System 7, it became separately-launchable control panel of its own.

Usage
For users of classic Mac OS, this control panel was useful for Macs containing, or attached to, more than one bootable drive, such as a CD-ROM, floppy drive, or hard drive. The first (or selected) icon in this control panel represents the volume containing a System Folder that will be loaded upon the next reboot. A different startup disk can be selected by clicking a another icon. This method was often used to boot from a system installer disk (floppy or CD-ROM) instead of the default internal drive.

Transition to Mac OS X
Before Mac OS X arrived, there were few cases of installing more than one System Folder on one hard drive. Versions of the Startup Disk control panel prior to Mac OS 9.1 recognized only one "blessed" System Folder per volume. The Mac OS 9.1 version recognized, for the first time in Macintosh history, more than one System Folder on one hard drive, thus enabling a switch to and from different versions of the Mac OS, including Mac OS X, on the same volume.

In Mac OS X, this control panel was superseded by the Startup Disk preference pane of the System Preferences utility.