Windows update breaks GRUB on dual boot machines

It’s pretty widely known that major Windows updates may affect dual boot machines. This is what I’ve run into with my dual boot machine – Windows decided to install a major update and in doing that broke GRUB that managed my boot options.

Few google searches later I’ve run across many ways of addressing the GRUB issue, which seem to be stemming from various failures.

What I was greeted with was GRUB command line with the following message:

error: file ‘/grub/i386-pc/normal.mod’ not found

and the prompt:

grub rescue>

In short, the following is the sequence of steps I needed to take:

1. First, run this command to see the list of disks and partitions:

   ls

   • You’ll get a list of devices and partitions in the form of: (hd0), (hd0,msdos1),etc.
2. Most likely your GRUB root partition is on (hd0)
3. You need to figure out which partition that is.
   • Run the following command:

     ls (hd0,msdos0)

   • If you get “error: unknown filesystem”, then move onto the next partition –

     ls (hd0,msdos1)

   • If you get “Filesystem is ext2” then you found one of Linux partitions – you may have more than one on a single drive, obviously. Most likely your root partition is the one with the smallest index.
4. Run the following commands to run GRUB (assuming your root partition is msdos1):

   set boot=(hd0,msdos1)
   set prefix=(hd0,msdos1)/boot/grub
   insmod normal
   normal

5. At this point you should see your GRUB screen, which allows you to pick the OS to boot.

However, this is not all. Following these steps you won’t “fix” the issue – you’ll just manually run GRUB. To permanently fix the issue and make sure you don’t have to follow these steps on every single boot. Assuming your root partition is on /dev/sda, run the following (note that it’s all about the device, not a partition on it):

sudo grub-install /dev/sda
sudo update-grub

So, at this point your system should be back to normal.

One more comment – if you run into this while upgrading Windows to the next major version, it doesn’t make sense to finalize the fix until Windows is done with its reboots (it does it few times during the upgrade), as it may damage GRUB again.