Startup disk

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

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.

Mac OS X
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.