Is anyone dual-booting both an older & newer Ubuntu version on a machine with nVidia graphics?

[Liam Proven]

I've encountered something weird, and I am wondering if it's some kind
of firmware issue.

I have 2 Thinkpads which both dual boot Win10 and Ubuntu 22.04. On
both, because I have the space, I have a 2nd copy of my Ubuntu root
partition; they're labelled "ubuntu_root" and "ubuntu_spare". I also
have /home and swap.

I adjust the settings in the spare (different fstab, different UUIDs
etc.) so they boot independently. Only the main copy controls the MBR.
They boot in legacy BIOS mode.

On one machine (with Intel integrated graphics only) I upgraded my
spare to the newer interim releases: 22.10, 23.04, 23.10. Then I hit a
problem: my nVidia 4200M GPU isn't supported in Mantic. The OS works
fine but I lose the GPU.

The problem is, once the 2nd partition was on a newer version, my old
partition won't boot to the login screen any more. X.org never loads.
"Failsafe graphics" in rescue mode doesn't work either.

I wasted most of today on trying to fix this. Ubuntu starts fine from
a USB key, my Windows partition is fine. Changing the firmware from
Optimus switching to either GPU makes no difference. I am baffled.


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