Windows 10 upgrade kills grub and boot-repair doesn't help

I just had to run this in Windows as an administrator:

bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi

Or, if you are using PowerShell:

bcdedit /set "{bootmgr}" path \EFI\ubuntu\grubx64.efi

GRUB appears fine on boot and I can boot into Ubuntu and Windows 10 fine.

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As pointed out in the comments (by David Faure), the final argument to this command depends on your particular system configuration. You can use this command to determine the correct path for your system:

bcdedit /enum firmware

This will list all the installed boot managers and associated EFI paths, and grub should be somewhere in the list.

The only solution that worked for me is described on that link I've brought it again here just because it's easier to find this thread on google.

Essentially the steps are:

1. Using Ubuntu Live to boot in 'Try Ubuntu' mode.
2. Use GParted to verify the correct linux partition (e.g. /dev/sda2).

3. Mount the linux partition:

   sudo mount /dev/sda2 /mnt  #Replace sda2 with your partition number
   

4. Mount all other linux system folders :

   for i in /sys /proc /run /dev; do sudo mount --bind "$i" "/mnt$i"; done
   

   If you are running an UEFI system, you will also have to mount your EFI directory using the following command:

   sudo mount /dev/sda1 /mnt/boot/efi
   

5. chroot into your install

   sudo chroot /mnt
   

6. Now that you are logged in your installation (not the Ubuntu Live) just do

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

And that's it ! Credits go do Scott Severance on the link I've mentioned above.

same problem here, upgrading from windows7 to 10 in a dual boot pc... I resolved in this way:

1. Upgrade Windows
2. During installation process, after the first reboot, i get the grub rescue prompt, fixed this with BootRepair in a USB stick
3. Completed the Windows installation, GRUB has gone and with a live cd of gparted i noticed that all partitions was still there but all of them were NTFS
4. Used TestDisk from windows recovered my Linux partition
5. BootRepair again and everything seems ok (i've Windows voice in GRUB menu duplicated.. i don't know why, but everything seems ok to me now!)

I hope it can be useful to someone!