HP Z420 not booting Linux from USB

I've got a HP Z420 workstation which won't boot Linux (GRUB) from USB. I've tried multiple USB ports (both USB 3.x and USB 2) and it gives the non-system- disk error. The USB stick in question has been used on 4 different Dell servers in the past few weeks (with 2 different Dell servers in the last few days) and works well. I booted the HP Z420 with a FreeDOS install USB stick and that works as expected, so it is booting from USB. Any ideas as to why a system might want to boot FreeDOS but not GRUB? -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/