Recover From Grub Failure

During to the upgrade from 3.x to 4.x, I found myself without a working grub and unable to boot. Monitor shows:

• grub rescue >

I attempted to use the proxmox 4.1 install disk, but found it had a bug where the prompt would not accept input.

You'll need a ISO for a 64 bit version of Ubuntu or Debian Rescue CD, must be 64 bit.

Boot Ubuntu/Debian off the ISO. We do not want to install Ubuntu/Debian, just run it live off the ISO/DVD.

First We need to activate LVM and mount the the root partition that is inside the LVM container.

• sudo vgscan
• sudo vgchange -ay

Mount all the filesystems that are already there so we can upgrade/install grub. Your paths may vary depending on your drive configuration.

• sudo mkdir /media/RESCUE
• sudo mount /dev/pve/root /media/RESCUE/
• sudo mount /dev/sda1 /media/RESCUE/boot
• sudo mount -t proc proc /media/RESCUE/proc
• sudo mount -t sysfs sys /media/RESCUE/sys
• sudo mount -o bind /dev /media/RESCUE/dev

Chroot into your proxmox install.

• chroot /media/RESCUE

Then update grub and install it.

• update-grub
• grub-install /dev/sda

If there are no error messages, you should be able to reboot now.

Credit: https://www.nerdoncoffee.com/operating-systems/re-install-grub-on-proxmox/