Startup Disk (System Preferences)

The Startup Disk pane of System Preferences in macOS was developed by Apple to allow users to select a bootable startup disk. When locked, access is limited to users with administrator privileges.

History
With the first release of Mac OS X, the Startup Disk preferences pane superseded the Startup Disk control panel from classic Mac OS.

Usage
Users can choose from a list of available, bootable versions of Mac OS X, such as an installation CD-ROM or DVD-ROM. Mac OS 9 can be selected in instances where older hardware will support it. Alternatively, a Network Startup volume can be selected. If there are many bootable folders to choose from, a horizontal scroll bar will appear.

The Restart... button can be clicked to reboot the Mac immediately after confirming in the dialog box that follows.

Other options
The Target Disk Mode... button can be clicked to have a Mac restart as an external drive through a supported interface, such as FireWire, Thunderbolt, or USB 3.0. However, this feature is being replaced with Mac Sharing Mode on Macs running Apple processors, starting with the Apple M1 in late 2020.