Step-by-step command list for remotely converting a FreeBSD
5.3-STABLE system from a single-disk/single-slice (ad0s1) to a
two-disk/single-slice (ad0s1 & ad1s1) GEOM mirror (gm0s1) based setup
without the need for console access or fixit/livefs CDROM:

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# make sure the first disk is treated as a really fresh one
# (not really necessary, but makes procedure more deterministically ;-)
dd if=/dev/zero of=/dev/ad1 bs=512 count=79

# place a PC MBR onto the second disk
# (with a single FreeBSD slice /dev/mirror/gm0s1 as large as the /dev/ad0s1)
fdisk -v -B -I /dev/ad1
size=`fdisk ad0 | grep ', size ' | head -1 | sed -e 's;^.*size \([0-9]*\).*$;\1;'`
(echo "p 1 165 63 $size"; echo "a 1") | fdisk -v -B -f- -i /dev/ad1

# place a GEOM mirror label onto first slice of second disk
# (actually on the last block of the disk)
gmirror label -v -n -b round-robin gm0s1 /dev/ad1s1

# activate GEOM mirror kernel layer
# (makes the /dev/mirror/gm0s1 device available)
gmirror load

# place a BSD disklabel onto /dev/mirror/gm0s1
# (ATTENTION: the /dev/mirror/gm0s1 device has to be specified as just
# "mirror/gm0s1" or the bsdlabel(8) will use the incorrect GEOM name
# "gm0s1" instead!)
# (NOTICE: start "a" partition at offset 16, "c" partition at offset 0)
bsdlabel -w -B mirror/gm0s1
bsdlabel -e mirror/gm0s1

# manually copy filesystem data from first to to second disk
# (same procedure for partitions "g", etc)
newfs -U /dev/mirror/gm0s1a
mount /dev/mirror/gm0s1a /mnt
dump -L -0 -f- / | (cd /mnt; restore -r -v -f-)
newfs -U /dev/mirror/gm0s1d
mount /dev/mirror/gm0s1d /mnt/var
dump -L -0 -f- /var | (cd /mnt/var; restore -r -v -f-)
newfs -U /dev/mirror/gm0s1e
mount /dev/mirror/gm0s1e /mnt/usr
dump -L -0 -f- /usr | (cd /mnt/usr; restore -r -v -f-)

# adjust new system configuration for GEOM mirror based setup
cp -p /mnt/etc/fstab /mnt/etc/fstab.orig
sed -e 's/dev\/ad0s1/dev\/mirror\/gm0s1/g' </mnt/etc/fstab.orig >/mnt/etc/fstab
echo 'swapoff="YES"' >>/mnt/etc/rc.conf
echo 'geom_mirror_load="YES"' >>/mnt/boot/loader.conf

# instruct boot stage 2 loader on first disk to boot
# with the boot stage 3 loader from the second disk
# (mainly because BIOS might not allow easy booting from second ATA disk
# or at least requires manual intervention on the console)
echo "1:ad(1,a)/boot/loader" >/boot.config

# reboot system
# (for running system with GEOM mirror on second disk)
shutdown -r now

# make sure the first disk is treated as a really fresh one
# (also not really necessary, but makes procedure more deterministically ;-)
dd if=/dev/zero of=/dev/ad0 bs=512 count=79

#  switch GEOM mirror to auto-synchronization and add first disk
#  (first disk is now immediately synchronized with the second disk content)
gmirror configure -a gm0s1
gmirror insert gm0s1 /dev/ad0s1

#  wait for the GEOM mirror synchronization to complete
sh -c 'while [ ".`gmirror list | grep SYNCHRONIZING`" != . ]; do sleep 1; done'

#  reboot into the final two-disk GEOM mirror setup
#  (now actually boots with the MBR and boot stages on first disk
#  as it was synchronized from second disk)
shutdown -r now

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It is also possible to easily switch back to a single-disk non-GEOM-mirror
based setup running on the first disk only:

# revert to old system configuration
cp -p /etc/fstab /etc/fstab.orig
sed -e 's/dev\/mirror\/gm0s1/dev\/ad0s1/g' </etc/fstab.orig >/etc/fstab
cp -p /etc/rc.conf /etc/rc.conf.orig
sed -e '/^swapoff="YES"/d' </etc/rc.conf.orig >/etc/rc.conf
cp -p /boot/loader.conf /boot/loader.conf.orig
sed -e '/^geom_mirror_load="YES"/d' </boot/loader.conf.orig >/boot/loader.conf

# remove second disk from GEOM setup and remove GEOM mirror meta-data on it
gmirror remove gm0s1 ad1s1

# reboot
shutdown -r now

# remove GEOM mirror meta-data from first disk
$ gmirror clear /dev/ad0s1
$ gmirror clear /dev/ad1s1