Two SSD, Two Linux Distro - Which grub is showing the grub menu during boot?

It looks like only Mint's GRUB is currently being used.

If you go to the firmware boot settings menu ("BIOS settings"), you may find that the current primary boot target is named just ubuntu instead of something like "Disk 1". UEFI allows the boot settings of installed OSs to have a clear human-readable name, and in this case it's ubuntu.

Since both Mint and Zorin are derivatives of Ubuntu, both of them apparently default to installing their GRUB to the /boot/efi/EFI/ubuntu directory on the ESP, so the last one installed will "win" by overwriting the bootloader of the other one.

BootCurrent: 0004 shows that the Boot0004 is the configuration that booted the currently running OS instance. The PARTUUID 264abb67-bc63-46f7-8106-01f8aa3c65d2 refers to sda1, which is the EFI System Partition used with the boot configuration Boot0004. The firmware will initially load /boot/efi/EFI/ubuntu/shimx64.efi, which is the Secure Boot compatibility shim which is signed by Microsoft. It will load and verify /boot/efi/EFI/ubuntu/grubx64.efi.

(The filesystem mounted to /boot/efi is FAT32, and according to the UEFI specification, it's supposed to be case-insensitive. However, some buggy UEFI firmware versions are case-sensitive. Here I'm assuming your firmware does not have such bugs.)

The firmware alone will accept bootloaders signed by either the hardware manufacturer or by Microsoft; the shimx64.efi adds Canonical's signing key and optionally the Machine Owner's Key (MOK) to the allowed list (non-persistently, so shimx64.efi must be executed in every boot). With Secure Boot enabled, you will need to set up MOK if you wish to build your own custom kernels or use third-party kernel modules.

(If you need to set up a MOK, see this article on Debian Wiki. It also applies on Ubuntu and derivatives.)

/boot/efi/EFI/ubuntu/grubx64.efi is a version of GRUB with essentially all GRUB modules built in and signed by Canonical to comply with Secure Boot requirements It comes from package grub-efi-amd64-signed. Because it's signed by Canonical, not by Microsoft, Canonical will be able to provide security updates to GRUB in a timely manner if required.

(When Secure Boot is in effect, GRUB won't be able to load GRUB modules as needed, since a Secure Boot-compliant firmware will mark all data that is not loaded from validly signed Windows-style PE+ binaries as non-executable for the CPU. Since GRUB modules use Unix-style ELF binary format instead of PE+, the firmware will not know how to check them even if the modules are cryptographically signed.)

There is probably a minimal GRUB configuration file at /boot/efi/EFI/ubuntu/grub.cfg that only contains the minimum number of configuration lines needed to access the filesystem that contains the /boot directory and load the real /boot/grub/grub.cfg from there, identifying the filesystem by an unique UUID. If you view it with e.g. sudo cat /boot/efi/EFI/ubuntu/grub.cfg, it will probably look like this:

search.fs_uuid 11111111-2222-3333-4444-555555555555 root set prefix=($root)'/boot/grub' configfile $prefix/grub.cfg 

In place of the string 11111111-2222-3333-4444-555555555555 will be the unique filesystem UUID of either sda2 or sde2: run lsblk -o +uuid and compare the UUIDs.

If you wish to change the location where the sda2's EFI/ubuntu/grubx64.efi reads its actual configuration, you can just edit /boot/efi/EFI/ubuntu/grub.cfg to point it to a different filesystem.

As the current OS seems to be Mint (= a derivative of Ubuntu), you might want to edit /etc/grub.d/40_custom on Mint to add an extra menu item to switch to Zorin's boot menu:

#!/bin/sh exec tail -n +3 $0 # This file provides an easy way to add custom menu entries. Simply type the # menu entries you want to add after this comment. Be careful not to change # the 'exec tail' line above. menuentry 'Switch to Zorin OS's GRUB menu' { search.fs_uuid <filesystem UUID of sda2 here> root set prefix=($root)'/boot/grub' configfile $prefix/grub.cfg } 

And when the Zorin OS is running, you can likewise add a menu entry to switch to Mint's GRUB configuration, in case Zorin's GRUB is ever used:

#!/bin/sh exec tail -n +3 $0 # This file provides an easy way to add custom menu entries. Simply type the # menu entries you want to add after this comment. Be careful not to change # the 'exec tail' line above. menuentry 'Switch to Mint's GRUB menu' { search.fs_uuid <filesystem UUID of sde2 here> root set prefix=($root)'/boot/grub' configfile $prefix/grub.cfg } 

If you want to have separate bootloaders for each OS, you must reinstall GRUB using a custom bootloader ID. To do that, on each OS:

• make sure /dev/sda1 is mounted as /boot/efi
• run grub-install --bootloader-id=mint or grub-install --bootloader-id=zorin respectively

This should result in two new instances of UEFI GRUB in directories /boot/efi/EFI/mint/ and /boot/efi/EFI/zorin/. These should also be registered as UEFI boot options named mint and zorin respectively. You will see the names in efibootmgr -v output; if your system's UEFI firmware is well designed, it will also show the names in the firmware boot order settings.

If you do this, once the boot options mint and zorin are both tested and working as intended, you may want to delete the old ambiguous ubuntu entry. That can be done with:

sudo efibootmgr -b 0004 -B sudo rm -rf /boot/efi/EFI/ubuntu