The correct way to restart or shut down a Macintosh computer is to choose the Restart or Shut Down commands from the Special menu in the Finder. When the computer starts up, a flag is set on the startup disk to let the Mac OS know that the computer started up correctly. When the computer is restarted or shut down correctly this flag is cleared. If the computer was not restarted or shut down correctly this flag is not cleared, for example: the computer is turned off using the power switch, or reset using the reset button.
When a Macintosh computer starts up this flag is tested. If the flag is clear the computer starts up normally. If the flag is set, the Mac OS knows that the computer was not restarted or shut down correctly. In this case, the computer rebuilds the volume bitmap. On a large startup disk with many files, this operation may take several minutes. Depending on the version of Mac OS installed on the startup disk, Disk First Aid checks the hard disk and repairs any errors that are found.