I had a dual-boot EFI-based Windows/Linux system, with grub2 pointing to an Ubuntu 18.04 /root partition in /dev/sda6.
I tried installing Ubuntu 20.04 in /dev/sda5, which was a spare partition. This install went okay, but hung on the grub-install phase. After that, grub no longer works. I just get a grub> prompt. I can escape out to the boot manager and boot Windows, but grub is hosed.
So, I booted the live Ubuntu 18.04 flash drive and attempted to rerun grub-install for the original 18.04 system in /dev/sda6 by mounting /dev/sda6 on /mnt and running
sudo grub-install --boot-directory=/mnt/boot /dev/sda
That resulted in this message
Installing for i386-pc platform. grub-install: warning: this GPT partition label contains no BIOS Boot Partition; embedding won't be possible. grub-install: warning: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged.. grub-install: error: will not proceed with blocklists.
Is there any way to fix this. It looks like something has messed up my partition table (or something), but the partitions themselves are okay. I can mount them and examine them under the live USB. I specifically asked UBU 20.04 to install grub in the partition device (/dev/sda5) rather than on the disk device (/dev/sda). Was that wrong?
I suppose, as long as /home is preserved (it's in /dev/sda8), I'm okay with something that wipes out my old 18.04 /root...