# allows gmirror to bypass chicken-egg problem (write metadata on rw mounted disk)
sysctl kern.geom.debugflags=16
# store metadata on first disk
gmirror label -v -b round-robin gm0 /dev/ad0
# be sure that kernel module is loaded
echo geom_mirror_load="YES" >> /boot/loader.conf
# change /dev/ad0* to /dev/mirror/gm0* with
vi /etc/fstab
# (or with)
sed -e 's/\/dev\/ad0/\/dev\/mirror\/gm0/g' /etc/fstab > tmp.$$; mv tmp.$$ /etc/fstab
# reboot
reboot
# clear any preexisting metadata on second disk  
dd if=/dev/zero of=/dev/ad2 bs=512 count=1
# add second disk to mirror
gmirror insert gm0 /dev/ad2
# wait till 100%
gmirror status
# reboot
reboot