I have experimented some more with my Asus Z87 UEFI system and .imgPTN files, and it seems that the firmware will not provide the user with the UEFI boot option to UEFI-boot from an MBR-partitioned USB drive, if the partitions on that drive are out of order.
The Partition Table in the MBR (first sector) has 4 'slots' for 4 partition entries.
Each Partition Table entry has the following fields:
The Partition Table in the MBR (first sector) has 4 'slots' for 4 partition entries.
Each Partition Table entry has the following fields:
- Drive number and Active/boot flag
- Partition Type number (e.g. 0C = FAT32)
- Start address of partition
- Number of sectors in the partition