Sophos Safeguard 5.60 client POA Boot from External Medium USB WINPE

Hi there,

We have a small form factor pc with no cdrom drive: Optiplex 780 Bios A06, and it has the BCD Boot error.

We have downloaded the latest WINPE iso from here http://www.sophos.com/support/knowledgebase/article/108805.html

"WINPE SGN 5.60.0.192 (based on WinPE 3.0) works for the SGN client 5.60.0.192 and 5.60.1.7. "

and we have the iso burnt to a CD. When our machines with cd roms have boot issues, we insert the cd. Authenticate POA and change the boot device to "External Media". It then authenticates and prompts us to press any key to boot from cd, and away we go. 

As this machine has no cdrom we have created a usb boot disk using the guide:

Building a Standard Image of Windows 7: Building a Standard Image of Windows 7: Step-by-Step Guide

Section: Build Windows 7 Media 

http://technet.microsoft.com/en-us/library/ee523217(WS.10).aspx however we used the iso's boot files not the ones mentioned in the guide .

The usb image works fine when booting from BIOS, we get the A43 Explorer GUI as expected.  When booting from POA external media however nothing happens it skips through too booting the hdd which gives us the windows boot error. I have changed the bios boot order, chose boot from floppy, toggled Shift F3 = switch USB Legacy support (Off/On)* .

Any ideas how to boot from External Medium at the POA and it not be a CD? From my reading this should be possible.