I have a working multiboot system with Ubuntu 18.04, Ubuntu 20.04, and Windows 10. The bootloader is Grub, which gives me all 3 options when I boot. Now I want to add Fedora 33. I have a 50 GB unused partition for this. I was not able to figure out the Fedora installer. Designating the partition for mounting “/” worked easily enough, but I could not figure out how to specify the boot partition, and the error messages were no help. I think I should simply leave it out, boot to Ubuntu, and do “sudo update grub” and let it find the new OS and add to the boot menu. So how do I do this? Or should I do something else?
I assume you already are sharing the efi partition? If not it is not really an issue except for the auto-detect feature of the install with additional OSes to boot.
With F33 I am told you don’t need a separate /boot partition when using the fedora grub since that version of grub is able to boot from the default btrfs file system. You must, however, mount the efi partition at /boot/efi. You also, if you wish, can have a separate /boot partition as ext4 with the remaining file systems as btrfs. The default structure for F33 is btrfs, with the main volume at / and one subvolume at /home.
So the default required structure is /boot/efi for the efi partition and / as the root volume and /home as one subvolume. You are able to do as many subvolumes as you require but that seems extra admin overhead. You also can choose to not use btrfs and instead remain with ext4 and LVM. That means, if you wish, that you could mount your /home from one of the other OSes instead of creating an additional space for that, while still having / as a new btrfs volume.
Fedora is able to detect additional installed OSes on a shared efi partition and configures grub accordingly.
Thanks, I will have another go.