The system I was trying this on was on an MSI neo2 platinum motherboard with
1 hd, 1 optical etc-a working desktop system.
I gave up trying to get it to work and thought I would try the usb stick on
my final hardware - an old shuttle sn41 with everything stripped out.
The key worked first time and booted monowall - seems to be working fine,
though I havn't really tested anything bar booting and checking the network
interfaces were detected.
Hope this helps someone else, as I was convinced this was a software
problem. Was trying to overcomplicate things. My msi motherboard has booted
off various usb devices no problem in the past, though first time I had
tried a *nix/bsd distro off a thumb drive specifically.
Didn't realise the booting was/could be sketchy on different hardware that
both supported various usb boot devices.