----- Original Message -----
> From: "Peter Olson" <peabo@???>

> So, I installed the Devuan beta a few days ago and it is great, rather it almost
> mostly works. I probably need an audio driver installed. I want to see what
> the old system had installed.
>
> So I cleared out another partition and moved the backup of my Debian 8.3 onto
> it. Ran update-grub, which found the backup in its new location.
>
> But, when I try to boot it grub is confused and is pinned to the old UUID of the
> root filesystem. (I have already updated /etc/fstab in the restored backup, but
> it is not even getting that far.) It just dumps me into busybox saying it can't
> find the root fs. Gotta love grub, which is useful only when nothing is wrong
> :-)
>
> Any advice about how to proceed?
>
If the other advice you got does not work, download super grub2 disk and boot from
that. It will find all your bootable OS's.

http://www.supergrubdisk.org/super-grub2-disk/