I have been working on (around) this exact same situation for about 2 mos.
My initial incentive was that I bought a nice laptop for cheap. But it lacked the
CD/DVD/Floppy adapter, or a Docking station. I was determined to get FBSD on it
immediately. So I used PXE. But to the point; it had a USB port that should
provide the ability to boot from (CD/DVD/Floppy/USB stick) because the option shows
up in the BIOS config - but no joy.
So after long research, I decided on GRUB (3 versions). The version you choose will
be best decided by which OS you currently have on the harddrive. Although you could
create a (20Mb) partition at the beginning of your first HD for GRUB and simply make
it the boot (active) partition. The trick then is to use FBSD's MD(4) (Memory Disk)
after you create one, and boot from it. You'll be able to "see" the USB drive and
choose it (chainload). I have little background (none) on your particular system, and
ultimate goal. But if you do any research on GRUB, you should easily discover which
version works best for your situation, and recipes as well. Hint; if you make a 20Mb
partition at the beginning of your hard drive, put GRUB4DOS in it, and make it the
active partition. You will have little trouble adding your startup MD(4)
files to it and adding your chosen startup partitions to the GRUB menu. Very easy - really!
HTH
--Chris