How to dual boot Debian 12 with existing another Linux installation by manual partitioning?

[Sheila_Flanagan]

@ihasama

My work laptop has btrfs filesystem with Gentoo which controls the grub.

That is the key. As I said earlier, having a btrfs OS control grub by installing it “after” MX Linux, I had no issues as it incorporated other installs into its grub. It is when you try to install the non-btrfs after the btrfs OS and have it control grub that issues arise.

If @Skywalker71 has already been using Fedora and does not want to install it again, we have to use a work around in order for Debian to show Fedora in its grub menu.

@Sheila_Flanagan

How to enable os-prober to detect btrfs installs with "/"-root on subvolumes...

For BTRFS installtions, where system root "/" files system is located on a btrfs-subvolume,
GRUB through update-grub ( => grub-mkconfig ) does have difficulties to generate
a GRUB menu entry for those other installations found on another partition.
update-grub is using os-prober. And os-prober does not have an indication
what subvolume "/"-root filesystem is located on.

In order to "help" os-prober to detect the GRUB-installtion on such btrfs-install
with subvolumes, some symlinks needs to be added to the "top id=5" subvolume.

So I had to add those symlinks and copy a folder from @/boot on that subvolume to the EFI partition of MX Linux. Then MX gained back control of grub and added the entry in grub menu for Garuda OS.

Sheila