A special shell will be launched by this option with a fixit Floppy
(or CD in later revisions of this utility) mounted as /mnt2.  This gives
one extra commands in /mnt2 as well as a more complete set of device
files in /mnt2/dev.  Some device operations, like fsck and disklabel,
may therefore require you to go to /mnt2/dev and use the entries there
rather than assuming that they will be present in the default /dev (which
came from the boot medium and is very minimal).

When you're done, exit the shell to reboot.
