Can't Boot to a USB Flash Drive -HP compaq dc7800p

I am trying to install FreeNas (FreeBSD based) OS onto my hp compaq dc7800p. For some reason I can't get the computer to boot to the flash drive. I turned on the "Removeable Media Boot" in the bios and set the flash drive to the top of the boot order.I was able to get the computer to boot to a flash drive and installed FreeNas onto the flash drive but now I am unable to boot from it. When booting from the flash drive I get the following image on the screen.