No ISO choice in boot menu

Hi,

I'm reimaging an NBU 5230 appliance and have the 2.6.0.1 ISO on a USB drive. I have put the USB into the back of the appliance but when the boot menu comes up (after an F6), it doesnt list a CDROM but just the USB drive, amongst the RAID controller and the EFI option.  I try to boot the USB anyway but it then tells me its an unrecognised filesystem and dumps me into the grub rescue prompt.

I download the ISO direct from fileconnect and when I check the ISO on a MAC or PC it looks fine.

Any ideas? Do I need to do something else to the ISO or anything in the boot menu of the appliance?