[Errata] Windows upgrade to 1703 broke grub

What happened?

After windows upgrading itself to release 1703 aka. the creator update, Grub bootloader can no longer start and says error: unknown filesystem and dropped in to grub rescue shell. Windows boots up fine from firmware memu.

TL; DR

Manually doing hexediting just taught me another valuable lesson: @Windows has the worst upgrade handling.

— @cth451 July 28, 2017

  1. Prepare a USB linux live environment.
  2. Find the correct partition number, it might have been changed during upgrade.
  3. Look for grubx64.efi in your EFI system partition and use a hex editor to open it
  4. Search for /boot/grub and notice the (,gptN) where N is your original partition number for linux OS.
  5. Change N to the new parition number, save the file and reboot.

Analysis

Before upgrade, my partition scheme was:

  • sda1 fat32: EFI system parition
  • sda2 ext4: Archlinux root filesystem
  • sda4 ntfs: Windows drive C
  • sda3 ext4: AOSC OS root filesystem

Partition numbers were not exactly in physical order of data, for I shrinked Archlinux to make room for Windows installation. However after upgrade, it became:

  • sda1 fat32: EFI system parition
  • sda2 ext4: Archlinux root filesystem
  • sda3: Windows drive C
  • sda4: Windows recovery environment
  • sda5: AOSC OS root filesystem

So what did windows do down there? It shrinked drive C and made a little recovery partition, then re-sorted the partition numbers. However grub bootloader looks for partitions by partition number. It tried parition 3 (which was a valid linux filesystem), but it’s an ntfs now.

This partition number is hardcoded into the bootloader efi image, so we need to manually hexedit it.

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