Introduction

This page describes a problem I found on my dual-boot system setup. It was briefly commented on the Arch Installation page. Both the explanation and the solution will be completely expanded in this page. The whole situation is a bit clowny but I hope this ends up being useful to someone in the same situation.

Problem description

With both operating systems (Windows and Arch) installed and correctly booting, I wanted to add the Windows boot entry to the GRUB installation done in Arch.

Context

System with two hard drives. Objective: install Windows in one drive and Arch on the other.

Windows installation problem

Windows installation problem description

Windows was installed in Legacy mode, booting in BIOS/MBR mode. This can be checked by:

• Booting into Windows
• Pressing WIN + R
• Running msinfo32.exe
• Checking the value of BIOS mode item (can be either UEFI or Legacy This made impossible for os-prober to detect windows. os-prober looks for an EFI system partition, however, the windows drive consisted of only one partition with everything.

Not a solution, but nice to have

Go to /etc/default/grub and uncomment:

# Probing for other operating systems is disabled for security reasons. Read
# documentation on GRUB_DISABLE_OS_PROBER, if still want to enable this
# functionality install os-prober and uncomment to detect and include other
# operating systems.
GRUB_DISABLE_OS_PROBER="false"

This will make grub-mkconfig -o /boot/grub/grub.cfg to run os-prober.

Along with that file modification it is also recommended to install ntfs-3g to be able to mount NTFS partitions without needing to explicitly specify the mount type.

pacman -S ntfs-3g

Windows installation problem solution

Format that drive and install Windows again ensuring that:

• The CSM (Compatibility Support Module) is disabled
• The installation medium was properly flashed. Using Rufus, select Partition scheme: GPT and Target system: UEFI(non CSM).

UEFI Arch entry missing problem

Why did the UEFI Arch entry dissapear?

To ensure that the Windows installation would not touch the Arch drive, I proceeded to disable that drive through the Motherboard UEFI GUI. Windows was now installed in UEFI mode.

However, the previous boot entry pointing to the Linux boot partition (when mounted: /boot/EFI/arch/grubx64.efi) was now missing and I was unable to boot into Arch anymore. I coult boot into the "drive" but that would do nothing as it points to the root partition, not the EFI system partition.

Adding the entry back

The steps to add the entry back were the following:

• Accessing the EFI shell. The EFI shell was accessed by selecting "Run UEFI shell from USB drive" under the exit section of the UEFI GUI and having the Arch flashed USB connected.
• Once in the EFI shell, identify the Boot Manager Device. This can be done using the map command.

It is a bit tricky because part of the output gets cut and I could not found a way to reduce the font size or do any kind of scrolling. I could manage typing fsX:\ where X is a number (e.g. 0,1,2,3,4) and then tabbing, looking for an autocompletion like fs3:\EFI\. Then, keep autocompleting until you can see the path fs3:\EFI\arch\grubx64.efi. In fact, if return is pressed with that path selected, you will boot into Arch!

• After doing what is described above and having located the entry, it can be added using the bcfg command to modify the UEFI NVRAM entries:

bcfg boot add 0 fs2:\EFI\arch\grubx64.efi "Arch"

• Then check the list of current boot entries:

bcfg boot dump -v

This should be it. After rebooting the system the boot option should show up in the GUI.

Solution to the initial problem

Finally, after booting back into Arch, with GRUB_DISABLE_OS_PROBER="false" running:

sudo grub-mkconfig -o /boot/grub/grub.cfg

did successfully detect and add the Windows entry to GRUB.