I know I am probably making a duplicate. But there are so many duplicates already that finding an answer to my specific questions will take hours. If not more. I tried.

So I succeeded in the dual boot. I have a ASUS FX504GM gaming laptop. Windows is on an NVME & the HDD now has DebianTesting.

My question.. did any of this at all effect the EFI partition of Windows NVME? I ask this because when I used to install grub on my old PC which had MBR disks and Legacy BIOS it would ask me which disk I wanted to install grub to.. /dev/sda or /dev/sdb. Idealy I would keep Windows in one disk. And install grub to the sdb that way I would be able to retain both Windows Boot Loader an grub on separate disks. This time .. grub did not ask which partition to INSTALL itself to. Now when I go into my UEFI setup (F2 at start) I see THREE boot option. Two of which are the Debian on disk ST1000LXSJS (Seagate Firecuda) and Windows bootloader on the NVME.

-Why are there two debian boot options?

-How does the EFI partition created on the Debian HDD work?

-Did grub install to two places?

Now here's the funny part.

I messed up the kernel updates and I am reinstalling Debian. I deleted the EFI and root partition on the HDD and my UEFI still shows the boot options.

So did grub add an entry somewhere in the NVME's efi partition?

  • You only need one EFI partition and the installer by default chooses the first drive. OSes coexist in the same EFI partition regardless of where they're actually installed. – user931000 Jun 3 '19 at 7:43
  • @GabrielaGarcia. So I can't have two EFI on two disks. Each for one OS? – BhooshanAJ Jun 3 '19 at 8:18
  • You can, sort of, but you shouldn't. It's not practical. You need to unlearn what you know about the old way (BIOS). Before only one bootloader could be installed in MBR so for dual-boot Grub would replace the Windows bootloader. Now, there's no MBR, the system boots from the EFI partition where all the bootloaders coexist peacefully. You still want to select Grub (Debian) as before but you can also boot Windows directly by selecting it in UEFI boot menu. – user931000 Jun 3 '19 at 8:25
  • No. My two EFI partitions are so that if by some weird chance... the NVME fails. I can still boot Linux from the EFI parition in the HDD. That said. I just installed EasyUEFI and saw that one of the bootloaders points to path:\EFI\DEBIAN\GRUBX64.EFI the other to -- path:\EFI\DEBIAN\SHIMX64.EFI. What is this? – BhooshanAJ Jun 4 '19 at 12:57
  • Yes, you can have two for that purpose. The different bootloaders are for booting with secure boot (shimx64) or without. – user931000 Jun 5 '19 at 1:54

Only one EFI partition is needed regardless of the number of physical drives present in the system. Installed OSes have their bootloaders in said partition regardless of where they are installed.

Having a second EFI partition in the second drive as in this case is not a requirement but it can be understood as a backup. In the eventuality of a failure in the main NVMe drive, having the second EFI partition in HDD will allow booting Debian, at least, without changing the firmware (UEFI) settings, typically, or in some cases, by moving the second drive up in the order (the first drive in order with a EFI partition is the one the firmware reads looking for bootloaders).

Lastly, the two bootloaders for Debian are for booting with Secure Boot (shimx64.efi) or without (grubx64.efi).

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