-isohybrid-gpt-basdat does nothing without an MBR template, so the ISO
had no partition table at all: written to a stick, it was a CD
filesystem at sector 0. AMI on a B550 (and OVMF) read El Torito off any
block device and booted it anyway. An HP all-in-one from 2012 looks for
an EFI system partition on a USB disk, as the spec says to, found none,
and listed the stick under Legacy only.
The FAT image now goes in a second time as MBR partition 2, type 0xEF,
and is FAT16 by cluster count rather than FAT12, the type old firmware
reads worst. Booted under OVMF both as a CD and as a USB disk. The ISO
also rebuilds when mkiso.sh changes.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4
125663929a