Hi,
From my remote position i would say that your firmware wanted
> In the end, I went ahead and manually compiled xorriso 1.3.4 and with
> this one and the command that Debian itself uses to create the
> netinst iso it worked (without the jigdo stuff). The resulting iso is
> bootable from the USB key.
to see the GPT rather than an MBR partition table.
So it would be EFI and not BIOS.
Whatever, congrats to your success.
Thomas
Have a nice day :)