this is obsolete doc -- see http://doc.nethence.com/ instead

Rescuing a FreeBSD 8 system with an FreeBSD 10 installation media 

Mount the root file system read-write, 

sudo su - 

dmesg | grep ^ad 

ls -l /dev/ada0* 

bsdlabel /dev/ada0s1 

fsck_4.2bsd /dev/ada0s1a 

#fsck_4.2bsd -y /dev/ada0s1a 

umount /mnt 

mount /dev/ada0s1a /mnt 

and proceed with your changes.