GRUB Repair Guide

Add Missing Windows 11 Entry (Tested on Debian 12 + Windows 11 UEFI systems)

==== 1. Basic Fix (90% Success Rate) ====


<code bash>
sudo apt install os-prober
sudo update-grub
</code>

==== 2. Manual GRUB Entry (For NVMe Drivers) ====

== a. Identify Windows EFI Partition ==

<code bash>
sudo lsblk -o NAME,FSTYPE,SIZE,MOUNTPOINT
</code>

Look for:
  * FAT32 partition (usually 100-500MB)
  * Example output: nvme0n1p1 vfat 100M /boot/efi

== b. Create Custom Entry ==

<code bash>
sudo nano /etc/grubd/40_custom
</code>

Add:

<code bash>
menuentry "Windows 11" {
   insmod part_gpt
   insmod chain
   insmod ntfs
   set root=(hd0,gpt1) #hd0 = first disk, gpt1 = first partition
   chainloader /EFI/Microsoft/Boot/bootmgfw/efi
}
</code>

== c. Apply Changes ==
<code bash>
sudo update-grub
sudo reboot
</code>


==== 3. Verify UEFI Mode ====

Linux should show UEFI vars:
<code bash>
ls /sys/firmware/efi/efivars
</code>

Windows partition should have EFI files:
<code bash>
sudo ls /boot/efi/EFI/Microsoft/Boot
</code>
Expected file is **bootmgfw.efi**


Troubleshooting Table

"hd0 not found"

  - Replace hd0 with actual disk number
  - Use lsblk to confirm partition numbers
No bootmgfw.efi

  - Check Windows installation mode (must be UEFI)
  - Disable "Secure Boot"
GRUB menu not showing

  - Hold Shift during boot
  - Run sudo grub-install /dev/nvme0n1