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http://lists.freebsd.org/pipermail/freebsd-questions/2003-April/002605.html
to enter Single User mode. At the question about the shell to use, hit ENTER. You will be dropped to a # prompt. Enter mount -u / to remount your root filesystem read/write, then run mount -a to remount all the filesystems. Run passwd root to change the root password then run exit to continue booting.
