
see 
http://www.freebsd.org/doc/en_US.ISO8859-1/articles/formatting-media/x76.html


2.3 Making Dedicated Mode Disks Using the Command Line

Execute the following commands, replacing ad2 with the disk name.

# dd if=/dev/zero of=/dev/ad2 count=2
# disklabel /dev/ad2 | disklabel -B -R -r ad2 /dev/stdin
We only want one partition, so using slice 'c' should be fine:
# newfs /dev/ad2c

However, the disklabel command above doesn't seem to work. The following is suggested:

fdisk -I /dev/ad3
bsdlabel -w /dev/ad3s1
newfs /dev/ad3s1a

I did discover that

newfs /dev/ad3c

would work without the disklabel or bsdlabel.