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Orian
Asked: 2017-03-25 18:36:11 +0800 CST

What to do when installing two distros on same drive

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I don't imagine this is the first time this has been asked. But, I have not hit on the right search string to find a definitive answer.

I wish to install Ubuntu Unity and Ubuntu MATE on the same SSD. I need Ubuntu Unity for reference system. (since I am still learning). And, MATE is simply because I have a strong affinity for that desktop environment.

I tried this on another system, and (probably to no one's surprise but my own) trashed the GRUB for the originally-installed distro... Fortunately, boot-repair got both working again. But, that cannot be the right way to do this... can it?

I have already installed UM on an 80GB Partition (sda2). The mount point chosen during installation is "/".

The SSD is partitioned with GPT partition table, as follows;

sda1 EFI Boot 512MB
sda2 80GB ext4 (Ubuntu MATE) 
sda3 80GB ext4 (Empty) 
sda4 328GB ext4 (shared data)  
sda5 2GB swap

Now, I wish to install the Ubuntu 16.04 Unity's file system on the other 80GB partition, sda3, without trashing the GRUB2 boot files for UM. My assumption was that it would "see" the other OS and just "add" itself to GRUB. But, that wasn't the case previously.

I cannot choose the "install beside" option because it wants me to change my partitions, which NO. I don't want to do...So, I need to do this through the "something else" option.

The questions are:

After choosing the location for the file system (sda3). Where should I tell it to put the boot loader (sda1 EFI, I assume) and what "mount point" should I choose during the Ubuntu 16.04 Unity installation? "/" is obviously already taken.

I really just want to make sure I am doing this in such a way as to ensure that both versions are properly recognized by GRUB2.

boot

1 Answers

  1. Best Answer

    Don't install Grub then. The default location would be indeed the EFI-partition, but Grub is already installed there from Ubuntu mate. This boot-loader would be overwritten. So don't install the boot-loader, but after installation of Ubuntu you have to boot to Ubuntu mate and run sudo update-grub.

    The option not to install the boot-loader does not exist any more in the something else-section of the installer. To install Ubuntu without the boot-loader boot your live-USB, choose Try Ubuntu, open a terminal and runubiquity -b or ubiquity --no-bootloader.

    There is a difference when it comes to the boot-mode (UEFI/legacy)

    In legacy-mode we would install the boot-loader to the MBR. But the installer will give us the option to install the boot-loader to the PBR (Partition Boot Record). This way the boot-loader in the MBR would not be overwritten during install of the second Linux-distribution. We could use the boot-loader in the PBR to create a chainload, but this is not really necessary. So, the first installed distro installs the boot-loader in the MBR, further installed distros install the boot-loader to the PBR or don't install the boot-loader are two clean solutions.

    In UEFI-mode the boot-loader is not installed in the MBR, it is installed in the ESP (System EFI Partition). As far as I know, the installer defaults to the ESP of the first drive (normally sda), even if we choose a different location. So all distros would install the boot-loader to the same location, overwriting an existing boot-loader which has been installed there before. As far as I see,the only way out of this is to install the second distro without boot-loader using the mentioned command above.

    Only one Linux-distribution needs to install a boot-loader, and only the system which installed the boot-loader should control the boot-loader. This single boot-loader is able to boot all other Linux-distributions. What happens if we have several Linux-distributions installed and all of them installed the boot-loader to the same location? I think, system-upgrades could lead to some trouble when the upgrade ships with a new version of Grub, the recent boot-loader in MBR or ESP might be overwritten and the boot-loader points to the system which performed the upgrade. This does not make the computer unbootable, but the layout of the boot-menu and the order of boot-menu-entries would change.

    One more hint for you: Whenever you delete the partition which holds the system which installed the boot-loader, you will run into problems. This partition holds the configuration-file /boot/grub/grub.cfg. The boot-loader needs to read this file, if you delete the partition which holds this file you will be greeted by Grub-rescue-mode the next time you boot.

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