I have a Debian bullseye installation on a luks fully encrypted drive that I wanted to copy to an identical drive and run on an Asus VC66-C (tiny form factor/no frills) during an upcoming trip. I had forgotten that the source machine is configured with the nvidia drivers, whereas the target machine is pretty basic/non-invida. So, of course, I get an error when I boot on the target machine. Is there a way to fix this on the target machine, or will I have to switch back to the non-nvidia video driver on the source machine, and then redo the "dd" copy (several hours) to the target machine? Maybe something I can do in the boot menu? I have no experience at this level of tinkering.

The boot menu was the answer, of course. I chose the second line and booted into recovery mode. I did several things, but I think these are the important ones:

1) In /etc/modprobe.d/nvidia-blacklists-nouveau.conf, remove the line that says "blacklist nouveau".
2) If you've used nvidia-settings to create an xorg.conf, then you must remove "/etc/X11/xorg.conf". If you don't remember, just look at the top of the file, and if it says it was created by nvidia-settings, then it needs to be removed.

I also used "#" to disable all the lines in /etc/modprobe.d/nvidia.conf, but I probably should have just removed the file.
The same is true for /etc/modprobe.d/nvidia-kernel-common.conf.
Also for /etc/modprobe.d/nvidia-options.conf

3) Run "sudo update-initramfs -u" to reconfigure the kernel with nouveau and without nvidia.

Then reboot and it should work under nouveau.