Hi, some remarks in hindsight about UEFI specs in regard to partitions:
The UEFI specs can be found at https://uefi.org/specifications with 2.11 being the current version (they generall stay backwards compatible). See especially https://uefi.org/specs/UEFI/2.11/05_GUID_Partition_Table_Format.html#legacy-master-boot-record-mbr Van Snyder wrote: > From fdisk -l: > ... > Disklabel type: dos > ... > Device Boot Start End Sectors Size Id Type > ... > /dev/nvme0n1p8 * 193429504 194545663 1116160 545M ef EFI > (FAT-12/16/32) UEFI 2.11, section 5.2.1, mentions MBR partition table as usable for marking an UEFI system partition. "If an MBR partition has an OSType field of 0xEF (i.e., UEFI System Partition), then the firmware must add the UEFI System Partition GUID to the handle for the MBR partition [...]" Some partition editors report this type as "EF00". But this does not mean that it has more than 1 byte of field width in the partition table entry. Section 5.2.2. specifies: "0xEF (i.e., UEFI System Partition) defines a UEFI system partition." This is what bootable Debian ISOs for amd64 use to advertise their system partition towards EFI. The rules for recognizing a valid MBR do not exclude logical partitions, but the C structure MASTER_BOOT_RECORD by which UEFI publishes a detected MBR partition table has only 4 elements in its MBR_PARTITION_RECORD array which is followed by another field named "signature", which must contain the bytes 0xaa55. So the 4-element array cannot be enlarged while keeping the struct compatible. I.e. MBR partition 8 is quite surely out of reach. All in all MASTER_BOOT_RECORD and the four MBR_PARTITION_RECORD elements represent the 512 bytes of a MBR, but not the bytes which describe logical partitions. (Those bytes are in the extended partition, not in the first block of the disk.) So it seems that logical MBR partitions are indeed unsuitable for marking the UEFI system partition. Another problematic aspect of /dev/nvme0n1p8 in above partition table is that its "BootIndicator" flag is set. UEFI 2.11 table 5.2 says: "This field shall not be used by UEFI firmware." In practice, some EFI implementations interpret this "shall not" as "must not" and refuse to use such a partition entry. Others seem to simply ignore this bit. ----------------------------------------------------------------------- Outside the specs, Microsoft has set the heuristic standard that any partition with a FAT filesystem shall be considered a potential EFI system partition, marked by the presence of a program named \EFI\BOOT\BOOT*.EFI where "*" is a CPU-type specific name part. (See Table 3.2 in https://uefi.org/specs/UEFI/2.11/03_Boot_Manager.html#removable-media-boot-behavior for CPU types and their boot program names.) Pete Batard, author of the program Rufus, stresses this fact when trying to convince producers of bootable ISOs to publish the content of the EFI system partition also as sub tree /EFI/BOOT in the ISO 9660 filesystem (modulo upper case / lower case). If so, one may copy all files from the ISO filesystem into a FAT filesystem on a USB stick and hope that this stick will boot via EFI. Have a nice day :) Thomas
