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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\/gm0/dev\/ad0/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.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 gm0 ad1
gmirror clear /dev/ad1

# reboot
shutdown -r now

# remove GEOM mirror meta-data from first disk
$ gmirror clear /dev/ad0

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ISSUES:

o second disk has to be equal in size. If it is smaller you have to
  reduce the size of the partitions in bsdlabel. If it is larger you have
  to make sure the GEOM mirror used just as much as the first disk is
  large (else you cannot add the first disk back later into the mirror)