Windows 10 updates can break GRUB bootloader: how to resolve

October 30, 2020

If you have had a dual boot computer booting Windows and Linux long enough, you have probably encountered this dreaded problem- Windows 10 updates and reboots, and suddenly you get a GRUB bootloader error screen that will not let you boot into Windows or Linux. The error looks like this:

error: unknown filesystem
grub rescue>

It happened recently to our techs, and we were left scrambling to find a solution. Well, we found a quick and easy one courtesy of user "Daniel" at AskUbuntu.com (we use Kali so this solution appears to be relevant to more distros than just Ubuntu). Daniel told us to do this:

Run ls command

ls
 gave: (hd2) (hd2,gpt6) (hd2,gpt5) (hd2,gpt4) (hd2,gpt3) (hd2,gpt2) (hd2,gpt1)

Now run ls (hd2,#) where the hashtag is a number between 1 and 6 (note: one can leave out the "gpt"-part)

You will get an error each time except for one number, which will be your root partition. In our case, ours was 5. So we ran these commands next (If your drive is shown in the ls command as hd0, then you would put hd0 in the commands below):

set prefix=(hd2,5)/boot/grub
set root=(hd2,5)
insmod normal
normal

The last command got us back into grub so we could boot into Kali. Then, per Daniel, we had two more things to do:

sudo update-grub


and when that completed:

sudo grub-install /dev/sda


After this we were able to reboot and get back to our normal GRUB boot manager. Keep this bookmarked for those future Windows updates!

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