The Startup Disk control panel appeared as of System 4.2 (then called Startup Device) and was integrated (please verify) into the Control Panel. It became a control panel on its own beginning with System 7. The selected icon in this control panel is the valid System Folder that will boot up a Mac. A different volume (icon) can be selected by clicking a different icon. For users of the Mac OS prior to Mac OS 9.1, this control panel has little use. However, it is useful if a System Folder is used on an external device or a RAM disk.