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 >

You can use Proxmox installation ISO in verison 5.4 or newer, and select debug mode. On the second prompt you'll have the full Linux tools, including LVM, ZFS, ..., available. If you exit that prompt you will come to the installation screens, simply hit abort there.

Alternatively, one can use a 64 bit version of Ubuntu or Debian Rescue CD.

Boot Proxmox VE in debug mode, or the 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/