Strange bug when booting legacy bios and efi/uefi compatible usb grub drive

Question

So basically I want a portable bootloader on a usb drive that can be used to boot any of the computers/distros I have preconfigured to work with this grub install. I am testing on ubuntu to figure out the ins and outs, and because I have two separate installs of ubuntu with bios and efi installs.

I am trying to do this myself will using techniques that others have developed. This is my partition table as read out by gdisk.

Number  Start (sector)    End (sector)  Size       Code  Name
   1           10240          624639   300.0 MiB   EF00  EFI System
   2          624640          626687   1024.0 KiB  EF02  BIOS boot partition
   3          626688         9015295   4.0 GiB     8300  Linux filesystem

The table is a gpt/protected mbr, the efi partition is fat32, and the 3rd partition where grub is installed is ext4.

The command I used to install grub was

sudo grub-install --efi-directory=/mnt/efi --boot-directory=/mnt --removable

The reason I have only done a efi install so far is that I have read in a few seperate places that grub efi can support bios systems with this partition configuration.

Given the context above my question relates to a strange problem I am having. When i try to test the drive on my bios based system, the bios won't post if the drive is installed. If I put the drive in any other port, same issue. If i blow partition table away with dd, the bios successfully posts. If i recreate the partition table without a grub install the issue returns.

I have not idea what is going on with this particular partition table structure. Does anyone know what's happening here?

F.Y.I. This issue does not persist in my efi/uefi systems.

Answer 1

So I spent about an hour testing everything I could of think of and seem to have solved the issue.

• When i try to test the drive on my bios based system, the bios won't post if the drive is installed.

  • If I put the drive in any other port, same issue.
  • If I blow partition table away with dd, the bios successfully posts.
  • If I recreate the partition table without a grub install the issue returns.
  • If I use this drive on any uefi system, this issue doesn't exist.
  • If I remove the unusual offset of the first partition the issue persists.
  • If I move the efi partition behind the bios_grub, and main grub partition, the issue persists.
  • If I make the whole drive a single partition with a efi attribute flag the issue persists.
  • If I make the whole drive a single partition with a BIOS_grub attribute flag the issue persists.
  • If I test the drive with a single blank partition without any flags, the issue persists.
  • If I do any of the previous tests of 2 other drives from different manufacturers the issue persists.
  • If I create hybrid mbr by converting the boot partitions from gpt to hybrid mbr after creation, this solved the issue.

  Googled my issue with hybrid mbr as a search criteria. I found this article of someone doing what I attempted to do. https://www.slivermetal.org/2016/09/18/how-to-create-an-hybrid-uefi-gpt-bios-gptmbr-boot-usb-disk/

I hope this helps others trying to do weird stuff with their bootloaders.