Dual boot is not detecting Windows Operating system

Ask Fedora
1

Hello,
I have a side disk that has Windows 10 installed in order to run some games that I cannot run in linux because of the Anti-cheat system.

So I had a disk with windows installed I can change to it from bios but not from Grub boot.

Device         Start       End   Sectors   Size Type
/dev/sda1       2048    206847    204800   100M EFI System
/dev/sda2     206848    239615     32768    16M Microsoft reserved
/dev/sda3     239616 467267583 467027968 222,7G Microsoft basic data
/dev/sda4  467267584 468858879   1591296   777M Windows recovery environment

/dev/sdb: 931,51 GiB is also in gpt and btrfs for linux

Tried this config but does not work

### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.

menuentry "Windows Boot Manager" {
    insmod part_gpt
    insmod fat
    search --no-floppy --fs-uuid --set=root 7A67-D4ED # Adjust to your disk and partition
    chainloader /EFI/Microsoft/Boot/bootmgfw.efi
}
### END /etc/grub.d/40_custom ###

Below linux partition config.
Device       Start        End    Sectors   Size Type
/dev/sdb1     2048       4095       2048     1M BIOS boot
/dev/sdb2     4096    2101247    2097152     1G Linux extended boot
/dev/sdb3  2101248 1953523711 1951422464 930,5G Linux filesystem

Also have previously ran Dracut
dracut --regenerate-all --force

Thank you in advance.

2

Remove the custom entry you made in /etc/grub.d/40_custom.
Then try booting into linux and use sudo grub2-mkconfig -o /boot/grub2/grub.cfg.

This should run the os-prober as part of the process and automatically find the windows boot loader and make it part of the grub menu.

The results should look something like this

 sudo grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
Found Windows Boot Manager on /dev/nvme0n1p1@/EFI/Microsoft/Boot/bootmgfw.efi
Adding boot menu entry for UEFI Firmware Settings ...
done

Also note that you seem to have removed two important lines in that file. Lines 1 & 2 do not appear in what you posted, but are 100% necessary.

$ sudo cat /etc/grub.d/40_custom
#!/usr/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.

I also note the error seen by @pg-tips below.

3

Your Windows installation uses UEFI mode (there’s an EFI System partition) but your Fedora is in legacy BIOS mode (there’s a BIOS Boot partition but no EFI System partition).

So when you boot into Fedora’s GRUB, you’re in legacy mode and I suspect it can’t successfully chainload the Windows UEFI bootloader from there.

You could either:

  • Just accept switching with the BIOS boot menu rather than GRUB
  • Reinstall Fedora in UEFI mode
  • …or maybe there’s a less destructive way to convert Fedora to UEFI mode, if someone knows it…
4

There is some info on that, but seems complex and not easy.