Some old BIOSes (from before year 2010) attempt to parse the boot sector and refuse to boot it if it does not contain a bootable MBR partition. This is a problem if one wants to use GPT on this disk, because, from the BIOS viewpoint, it contains only one, non-bootable, MBR partition of type ee (i.e., the protective MBR partition). One can mark the protective MBR entry as bootable using fdisk -t mbr /dev/sda, and it will work on some BIOSes. However, the UEFI specification prohibits the protective MBR partition entry from being bootable, and UEFI-based boards do care about this, even in the legacy boot mode. So, this matters if one wants to create a GPT-based USB flash drive that is supposed to boot both on modern UEFI-based boards and also on old BIOSes that insist on finding a bootable MBR partition. It is not possible to solve this problem using traditional tools such as fdisk or gdisk, but it is possible to create a fake MBR partition entry suitable for both kinds of BIOSes manually as a sequence of bytes.

The command below will overwrite the second MBR partition slot and add a bootable partition there of type 0 (i.e. unused), covering only the first sector of the device. It will not interfere with the GPT or with the first MBR partition entry which normally contains a protective MBR partition.

printf ‘\200\0\0\0\0\0\0\0\0\0\0\0\001\0\0\0’ | dd of=/dev/sda bs=1 seek=462

The end result will look like this: fdisk -t mbr -l /dev/sda

Disk /dev/sda: 232.9 GiB, 250059350016 bytes, 488397168 sectors Disk model: ST3250820ASUnits: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0x00000000

Device Boot Start End Sectors Size Id Type /dev/sda1 1 488397167 488397167 232.9G ee GPT /dev/sda2 * 0 0 1 512B 0 Empty

Partition table entries are not in disk order.