The final code takes 399 bytes, so there is still free space for the disk label (bytes 440-443), read-only flag (bytes 444-445) and partition table (446-509).

The final code takes 399 bytes, so there is still free space for the disk label (bytes 440-443), read-only flag (bytes 444-445) and partition table (446-509). truncate -s 100M disk.img dd if=bootmine.img of=disk.img bs=512 count=1 conv=notrunc fdisk disk.img # n -> enter -> enter -> enter (new primary partition, use all space) # t -> c (set type to FAT32) # w (save) sudo losetup -f disk.img --show -P # it will print the device path, e.g. /dev/loop21 # -P is to auto-detect partitions sudo mkfs.vfat /dev/loop21p1 # Now you can use e.g. GNOME Disks to see that # the partition is "normal" (you can mount it, # put files there, etc). # Does it boot? sudo qemu-system-x86_64 -drive file=/dev/loop21 # Looks good to me, though I don't have time to play rn.