Boot from a FreeBSD CD and choose "Fix-it" mode
chroot /dist
mount_devfs devfs /dev
gmirror clear /dev/ad4
gmirror clear /dev/ad6
gmirror load
gmirror label -v -b load gm0 /dev/ad4
mount /dev/mirror/gm0s2a /mnt
echo geom_mirror_load="YES" >> /mnt/boot/loader.conf
echo swapoff="YES" >> /mnt/etc/rc.conf
sed "s%ad4%mirror/gm0%" /mnt/etc/fstab > /mnt/etc/fstab.new
mv /mnt/etc/fstab /mnt/etc/fstab.old
mv /mnt/etc/fstab.new /mnt/etc/fstab
gmirror insert gm0 /dev/ad6
See:
http://freebsdwiki.net/index.php/RAID1,_Software,_How_to_setup
gmirror list
