Dualboot on Debian

Hi. I have a Debian and Linux Mint installed. I would like to test, learn, install Rocky on my system as dual boot. Furthermore, I did try to install, but I couldn’t find the other distros which were installed before when I booted after Rocky installation. I couldn’t find any info on how to dual boot any Red Hat alongside Debian based distro too. Kindly help me with this. I know I can try VM, but I still would like to go with dual boot on this machine.
Debian is in sda2, LinuxMint is on sda4.
Kindly help.

Unless you need direct access to the hardware, I suggest that you use a VM.
It has at least two benefits:

  • you don’t end up with this problem (or worse, wiping the other OSs)
  • you can run both in parallell
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You seem to have legacy boot from MBR (since I see no ESP).

UEFI mode is really nice:

  • The related GPT allows 100+ primary partitions, where MBR is limited to four.
  • Boot loaders are in separate ESP (EFI System Partition), each in its vendor’s subdirectory.
    Therefore, while each distro shares /boot/efi, they can each have separate /boot and updates to one are less prone to mess the others.
  • One can choose the distro to boot with UEFI boot menu, although one can still have “other OS” entries in bootloader’s menu too

Alas, no idea whether one can convert MBR to GPT in-place for Debian and Mint, assuming the hardware supports it. (MS Windows miraculously has in-place conversion.)

As is, I do recommend the VM too.

What do you mean by you “couldn’t find them”?, they are all there in the partition manager

If one has legacy mode, then the firmware loads first stage of bootloader from sector 0 of (only) disk and that bootloader has rest in /boot/grub2 of the most recently installed distro. In other words there can be only one bootloader. One menu.

The logical question is: does the installer / os-prober of Rocky find and add entries for kernels of other distros from other partitions than where Rocky has /boot?

If it doesn’t, then one has to add those entries. Probably easiest is to add chainloader entries; to make Rocky’s GRUB load the bootloaders of those other distros. Then it is not necessary to maintain entries for each kernel of those “foreign” distros.

This seems to be a problem with the GRUB configuration, not really with Rocky itself right?
I would recommend this guide for a user friendly tutorial of how to configure GRUB when dual booting.

Nothing seems to be wrong, his current GRUB configuration is just not made to display all three entries.

Yes, and that is the point. Some distros – when they install – do insert into their GRUB config also other installed distros (or so I have heard). RHEL seems to do less so.

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