bootable 7.1 USB disk versus flash drive versus internal ide hard drive

[Fbsd1]

Is there a differences between a usb external hard drive
and a usb flash drive boot loader pgm?
When i plug a usb flash drive in FreeBSD handles it
just like a internal IDE hard drive.

To make a hard drive bootable you have to write the boot loader pgm to 
the disk. Does the ide hard drive boot loader pgm work on usb external 
hard drives and a usb flash drive?

My pc has bios option to boot from usb disk but will not boot when i 
plug in a bootable flash drive.

What am i missing here?