Startup disk

A startup disk, or startup drive or boot disk, is a volume which will allow a computer to boot normally. Without one, a Mac will not be able to start.

Apple II series
In an Apple II computer, if no bootable floppy disk with Apple DOS or ProDOS is detected, the system will default to Apple BASIC from the ROM. The Apple II plus would default to Applesoft BASIC.

Classic Mac OS
Under classic Mac OS, a valid startup disk has a blessed System Folder. This refers not only to a System Folder, but also one that has enough elements to warrant its use for starting up a Mac. Classic Mac OS will indicate whether the folder has been "blessed" by displaying a miniature Finder or Mac OS logo on the icon of the folder itself. A valid blessed System Folder must have at least the Finder and System file. Beginning with System 7.1, Enablers may also be needed, and as of the New World Macs (beginning with the very first iMac (G3), the Mac OS ROM file and Open Firmware related items are also required. A valid startup volume in classic Mac OS can be selected with the Startup Disk control panel.

Mac OS X and macOS
Under Mac OS X, due to its degree of system complexity, a full, properly-installed System folder is the only properly counted "blessed System folder" and thus constitutes a valid startup disk. However, by using APFS in macOS High Sierra (10.13) or later, it is possible to have multiple startup volumes on a single drive. A valid startup volume in Mac OS X or macOS can be selected through the Startup Disk pane in System Preferences.