Plug in usb flash drive to computer.
Look what we have on flash drive:

# gpart show

By default flash drive initialise in system like da0.
Destroy all data on flash drive:

# gpart destroy -F da0

Create partitions.

# gpart create -s gpt da0
# gpart add -t freebsd-boot -s 128 da0
# gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 da0
# gpart add -t freebsd-swap -s 512M da0
# gpart add -t freebsd-ufs da0
# newfs -U /dev/da0p3
# mount /dev/da0p3 /mnt
# exit

# vi /etc/fstab

/dev/da0p2 none swap sw 0 0
/dev/da0p3 / ufs rw 1 1