Grub issue

[rustleg]

I have installed 2 copies of Linux Mint with BIU. Each has a separate root partition and swap partition, both can access a common NTFS data partition (will use later with Windows in a VM). When I boot the second Mint OS it always starts with a Grub screen. I hit enter and it then boots the OS. The first one just boots straight into the OS without showing any Grub options. I am concerned that something's amiss with the second OS boot sequence.

Each OS has its own Boot File in the Boot Edit screen \EFI\ubuntu.001\shimx64.efi and \EFI\ubuntu.002\shimx64.efi respectively in the EFI system partition. But I don't really understand how the EFI system partition fits into the boot sequence.

I tried to fix it by modifying the grub configuration file /etc/default/grub in the second OS to a timeout of 0 seconds but this seems to be ignored. Also set GRUB_DISABLE_OS_PROBER=true as suggested by Perplexity AI. I did run update-grub.

Another issue, maybe related, is that the grub options include the first Linux OS although I have hidden its partition in the boot edit of the problem OS which I thought would stop it finding the first Mint OS.

[Brian K]

I tried to fix it by modifying the grub configuration file /etc/default/grub in the second OS to a timeout of 0 seconds but this seems to be ignored.

I noticed 0 seconds was ignored too. I use 1 second.

[rustleg]

This is a bigger problem than I thought. Grub sees the partitions I have marked as hidden.

When I run fdisk -l the supposedly hidden partitions appear with type "unknown". Is this normal?

It is important to me that each Mint system does not see the other. This is fundamental to why I am running BIU. Otherwise one which could be infected with malware could infect the other. I appreciate that the "unknown" partitions won't be mounted but in theory any software running in the OS could access those partitions.

[johnstrasser]

This is a bigger problem than I thought. Grub sees the partitions I have marked as hidden.

When I run fdisk -l the supposedly hidden partitions appear with type "unknown". Is this normal?

That's pretty normal. Generally they will be marked as unknown or blank. Remember, everything is working off the same gpt partition table.

So to play around, take a blank disk (in a vm or another computer) and, before you install mint, create a bunch of partitions (you can even format them). Then install mint.

fdisk, gparted, disks will *show* them all

[rustleg]

In a MBR system the boot manager BIBM rewrote the MBR so it hid the unused partitions and only showed the ones selected. The rest of the disc appeared as unused space so for these hidden partitions no partition information was available to the OS, which in my view made it a bit more secure against possible malware looking to infect other partitions. If it was possible for BIU to fiddle the gpt to omit the hidden partition information it would be better. I guess perhaps this would be dangerous for other reasons if anything goes wrong.