[Allow the mounted disk (ad0) to be modified by gmirror]
This step allows the gmirror utility to modify your drive while it is mounted and is a one time operation during gmirror setup, and will not be restored on reboot.
# sysctl kern.geom.debugflags=16
[Create the mirror on ad0]
This command modifies the drive label meta-data on your hard disk so that it is recognized as a gmirror RAID disk. In the example I set gm0 but you can give it any label you like as long as you’re consistent. You will need this later to modify your /etc/fstab.