GRUB cannot detect Windows 10

Hi. I have an issue where Windows 10 is not being detected in the grub menu. Here’s the state of my machine for background info:

  1. Installed Windows 10 fresh on an SSD (/dev/sda)
  2. Installed Manjaro KDE fresh on an SSD (/dev/sdb). During Installation, chose this same SSD as the boot loader location.
  3. Enabled grub menu by editing /etc/default/grub
  4. Ran the os-prober and update-grub

When I reboot, Windows is not being found by grub. However, I can boot into it from the BIOS. GRUB_DISABLE_OS_PROBER is already set to false and uncommented.

Running sudo efibootmgr returns “EFI variables are not supported on this system”. I don’t know if this matters.

I have seen some answers saying that I should manually add a Windows entry to grub, but I am a newbie so I have no idea how I would go about doing this. Should I do more research into that process or is there a simpler solution?

Welcome to the forum! :slight_smile:

Yes, it does. When installing two operating systems in dual-boot on the same drive, then both operating systems must boot up in the same manner, being either both in native UEFI mode or both in legacy BIOS emulation mode, but not one in UEFI mode and the other in legacy BIOS emulation mode.

It is always best to, before the install, disable the legacy BIOS mode in the UEFI firmware setup utility of your computer, and to disable Windows Fast Boot and Secure Boot.

Thank you for the warm welcome!

Just to clarify, I have Windows installed on SSD A and Manjaro on SSD B, but there was a separate option during installation to choose a boot loader location, which I put on disk B where my Manjaro partition was going. Does this change anything about what I should troubleshoot? Or would it still be good to reinstall Manjaro after disabling legacy BIOS, fast boot, and secure boot?

There is a way to switch over from legacy boot to UEFI boot, but it’s quite complicated and even I myself wouldn’t touch that stuff, so my advice would be to reinstall Manjaro in UEFI-only mode.

Just remember to use a GPT partition table ─ not “MS-DOS MBR” ─ and create an EFI system partition of about 512 MiB, with a FAT32 filesystem on it. Then create your Manjaro partition(s) behind that.

I just reinstalled Manjaro after disabling Legacy BIOS and everything seems to be working fine now. Thank you!

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