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Note: I feel the two questions below are basic questions, however I could not find a clear answer searching the web.

Background

I would like to install Grub2 in some free space after the MBR. I would like to not install it in a dedicated partition, to not waste a partition entry as I’m running a PC with a legacy partition model limited to four primary partitions. I will have two Ubuntu versions and some other OS which Grub2 is always able to boot using chain‑load.

Questions

When I run du -h /boot/grub on my actual installation, I see it uses about 7.2MB. Is this a typical space required for all Grub2 stages? Does that mean if partition with 10MB left free before the first partition, this will always be enough free space to install Grub2 in /dev/sda?

If I install Grub2 there, then can I be sure automated Grub update from any of the two Ubuntu installations will work properly?

Hibou57
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    BIOS only boots from the MBR. If you have more than one drive you have multiple MBR and can have different boot loaders in each MBR. Grub2 also uses some of the space after the MBR for core.img. With BIOS/MBR you can convert one primary partition to the extended partition which acts like a container for all the logical partitions you want. Install/use grub from the system you use the most into MBR. And either do not install grub or install to a throw away like to a partition where it will not be used. http://askubuntu.com/questions/848119/how-to-update-grub-on-a-dual-boot-machine/848614#848614 – oldfred Sep 16 '17 at 03:32
  • So only the core.img can be stored in the free space next to the MBR, not the other files? I’m not sure I have understood the part about conversion, I will search the web about it. Thanks for your comment, it’s a good starting point. – Hibou57 Sep 16 '17 at 05:21
  • Lots more info on grub: https://help.ubuntu.com/community/Grub2#A.2BAC8-etc.2BAC8-default.2BAC8-grub_.28file.29 & https://help.ubuntu.com/community/Grub2/Setup#Configuring_GRUB_2 – oldfred Sep 16 '17 at 12:16

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