A startup disk or startup driveis a volume which will allow a Macintosh or Mac OS computer to boot normally. Without one, the Mac won't start; hence the name startup disk.

A valid startup disk has a blessed System Folder. This refers not only to a System Folder, but also one that has enough valid elements to warrant its use for starting up a Mac. A blessed System Folder often has the Mac OS logo on the folder itself.

Under the Classic Mac OS, 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 items Mac OS ROM, System Resources and Open Firmware related items are also required.

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.

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