Comment 4 for bug 40993

Per my earlier comment I used apt to perform a dist-upgrade from 5.10 to 6.06 (beta) on this problematic system. I was starting from a fresh 5.10 installation. The upgrade went off without a hitch until the subsequent reboot. Then I encountered the same problem as I had previously. The boot was unable to find the root filesystem.
Recall that / is supposed to be mounted from /dev/sda1.

I dug a little bit deeper. In the busybox shell, the root filesystem was mysterously found on /dev/sdc1. On a lark, I decided to edit the grub commands and get root from /dev/sdc1. This allowed the boot to proceed.

The next problem I ran into was that /usr (which I had placed on /dev/md1 during the installation process) could not be mounted. It turned out that this partition was, post boot, known as /dev/md0.

By telling grub to get /root from /dev/sdc1 and editing fstab to get /usr from /dev/md0 I am now able to enter the system.

In summary:

During installation I define the following mount points

/dev/sda1 /
/dev/md1 /usr

I must tell grub that root is on /dev/sdc1 in order to boot.

The /etc/fstab file contains the mount points in order to successfully perform the initial mounts:

/dev/sda1 /
/dev/md0 /usr

Hopefully this is making sense to someone.

Additional information and experimentation available upon request.

Gisli