[FIX] Grub error: unkown filesystem

These holidays I’ve updated my laptop’s Windows with a dual boot to Windows 11.
As I expected something happened and after the first restart I got this message:

error: unkown filesystem
Entering rescue mode...
grub rescue >

The Windows update did something bad to my Grub and the selection menu did not appear.

After some research I got this fix from these resources:

https://www.partitionwizard.com/partitionmanager/grub-rescue-unknown-filesystem.html

https://www.diskpart.com/articles/grub-rescue-unknown-file-system-0825.html

https://slimbook.es/tutoriales/linux/164-3-maneras-de-como-reinstalar-o-reparar-grub-boot-repair-archlinux-o-antergos

First, you need to find out which partition is your Linux system installed and the which is the grub directory.

From the Grub rescue mode:

grub rescue > ls

This command will list all partitions available in the system.

(hd0), (hd0,gpt1), (hd0,gpt2), (hd0,gpt3) ...

grub rescue>ls (hd0,0)
Repeat this command for all partitions available until you get a answer diferent than “unknown filesystem”.

Now assuming the correct one is (hd0, gpt6):

grub rescue > set root = (hd0,gpt6)
grub rescue > set prefix = (hd0,gpt6)/boot/grub
grub rescue > insmod normal
grub rescue > normal

You will get the usual Grub menu.
Enter then into your linux system and fix Grub.

To fix I followed this method using a Linux liveUSB:

sudo add-apt-repository ppa:yannubuntu/boot-repair 
sudo apt-get update 
sudo apt-get install -y boot-repair 
boot-repair

Restart and your Grub menu should be back again.