@hv.fotech (big hd)
--
  mkdir /media/newimage
  cd /usr/src

-- ver1
  make DESTDIR=/media/newimage buildworld
  make DESTDIR=/media/newimage buildkernel
  make DESTDIR=/media/newimage installkernel
  make DESTDIR=/media/newimage installworld
  cd /usr/src/release/scripts
  ./make-memstick.sh /media/newimage /tmp/my-memstick.img

-- my ver
dd if=/dev/zero of=rescue.img bs=1 count=1 seek=1967028736
mdconfig -at vnode -f rescue.img
newfs -nUL rescue /dev/md2
mount -o noatime /dev/md2 /mnt

cd /usr/src
make buildworld buildkernel	KERNCONF=CPMAUSB
make installworld		DESTDIR=/mnt
make installkernel		DESTDIR=/mnt KERNCONF=CPMAUSB

cd etc
make distribution 		DESTDIR=/mnt

vi /mnt/etc/fstab
/dev/da0s1a	/	ufs	rw	1 1

umount /mnt
dd if=rescue.img of=/dev/da0
try new usb out

--
Insert usb stick
fdisk -BI /dev/da0
bsdlabel -B -w da0s1
newfs -U /dev/da0s1a
mount /dev/ad0s1a /mnt
make installworld installkernel distribution DESTDIR=/mnt

Edit /mnt/etc/fstab
/dev/da0s1a /      ufs       rw                                           1 1

Edit /mnt/etc/rc.conf for your needs
unmount /mnt


mkdir /usr/USB-or-CD

cd /sys/i386/conf
cp GENERIC CPMAUSB

[cpfong@hv /sys/i386/conf] tail -1 CPMAUSB 
options ROOTDEVNAME=\"ufs:da0s1\"