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I want to dual boot my computer. It has an EFI, not a traditional BIOS. It has two drives. I want to install Gentoo Linux on the first NVMe SSD (according to the EFI firmware). I currently have Windows 10 installed on the main drive (second NVMe SSD, according to firmware). I have my second SSD as the default boot drive (physical location to logical numbering of drive was an afterthought) with the whole drive used for the OS. After erasing what used to be my data drive, I am now ready to install my distro.

Since I want to leave the default boot drive setting on the firmware the same, I have a few choices to make as far as partitioning the new drive:

  1. Omit any boot partition and use the EFI partition of the main drive. Only issue is it would need to be resized, which would require the moving of around 800GiB of data over by around 412 MiB, creating a significant amount of wear on the drive.
  2. Create a FAT32 partition at the beginning of the new drive, setting the esp and boot flags with parted.
  3. Create a FAT32 partition at the beginning of the new drive, without adding any flags to the partition.

For the latter two options, I would use the FAT32 partition to store my kernel(s) and use a relatively lightweight boot-loader like rEFInd on the first drive (by default Windows 10 will only create an insufficient 100MiB partition for use for EFI, leaving little room for kernels).

Since I would prefer not to use the first option, my question essentially boils down to the following: Would setting (or not setting) the flags on the secondary drive (as in #2) mess with booting either OS? Would the EFI firmware be able to run a kernel (or GRUB2) from the secondary drive after loading the boot menu (rEFInd) from the main drive? The kernel will be executed directly by the firmware. It will have a .EFI file extension.

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  • Ubuntu does not make it easy, but I have an ESP on every drive and different boot loaders in each drive. Both Debian & Fedora let me install to my second drive's ESP. Ubuntu only wants to install to first drive's ESP and you have to do major workarounds to use anything else. I prefer to have an ESP on every drive, if only as a backup or to make drive available for an install later without major reformatting.
    – oldfred
    Dec 6, 2020 at 3:30

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On multi-boot systems where I have the systems installed on different drives I'm doing this usually by pysically disconnecting the drives I don't need for this system and do a default setup (which is a little more complicated on gentoo compared to distributions with installers). This also prevents accidentially deleting data from a drive which has another production system installed.

After the installation I use a drive which has a grub-configuration with os-prober to find the other systems. It's a long time since I used grub to load another grub but it's possible with chainloading. Compared to my memory you can see more recent posts here: (UEFI) Chainloading GRUB from GRUB

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