Quick howto – RAID-1 (mirror) in FreeBSD

sysctl kern.geom.debugflags=16

gmirror label -v -b round-robin gm0 /dev/ad0

echo geom_mirror_load="YES" >> /boot/loader.conf

sed -i .orig 's/ad0/mirror/gm0/' /etc/fstab

reboot

gmirror insert gm0 /dev/ad2

Replace ad0 with your FreeBSD bootdisk, ad2 with the second (same or larger capacity) disc.