I received the following boot error on an Ubuntu 16.04/Windows 10 dual boot PC after the Windows 10 Fall Creators Update was installed automatically by Microsoft;
error: unknown filesystem
Entering rescue mode
grub rescue> _
I solved it by a) running Ubuntu using the live CD (Ubuntu 16.04 installation disc) and b) reinstalling grub.
a) Run Ubuntu using the live CD:
Warning: modifying the bios can damage your system
- Ensure that your computer's bios is set to boot from the optical media drive first (if a disc is inserted)
- Insert Ubuntu Live CD, and restart computer
- Choose to start Ubuntu using the live CD (don't reinstall Ubuntu)
- If the Ubuntu live session is buggy (for example the compiz graphical user interface flashes), then ignore the user interface and open up a terminal directly (e.g. Ctrl-Alt-F3). Login to the terminal using username ubuntu and no password.
b) Reinstall (Repair/Restore) Grub:
I followed these instructions (from http://howtoubuntu.org/how-to-repair-restore-reinstall-grub-2-with-a-ubuntu-live-cd);
Warning: Using the sudo command, especially from a Live CD can do serious damage to your system. Read all instructions and confirm you understand before executing any commands. When pasting into the Terminal, use Ctrl+Shift+V, NOT Ctrl+V.
Find out what partition your Ubuntu Installation is on. It is usually an EXT4 (linux) Partition. Here are some methods to do this;
Mount the partition your Ubuntu Installation is on. Replace the XY with the drive letter, and partition number, for example: sudo mount /dev/sda1 /mnt.
sudo mount /dev/sdXY /mnt
Now bind the directories that grub needs access to to detect other operating systems, like so.
sudo mount --bind /dev /mnt/dev &&
sudo mount --bind /dev/pts /mnt/dev/pts &&
sudo mount --bind /proc /mnt/proc &&
sudo mount --bind /sys /mnt/sys
Now we jump into that using chroot.
sudo chroot /mnt
Now install, check, and update grub. This time you only need to add the drive letter (usually a) to replace X, for example: grub-install /dev/sda, grub-install -recheck /dev/sda.
grub-install --recheck /dev/sdX
Now grub is back, all that is left is to exit the chrooted system and unmount everything.
sudo umount /mnt/sys &&
sudo umount /mnt/proc &&
sudo umount /mnt/dev/pts &&
sudo umount /mnt/dev &&
sudo umount /mnt
Shut down and turn your computer back on, and you will be met with the default Grub2 screen.
This enabled grub to load properly and boot into Ubuntu. To enable grub to boot into Windows again, I had to c) detect the Windows partition and reupdate grub (from GRUB does not detect Windows);
c) Detect the Windows partition and reupdate Grub:
Detect the Windows partition: