"sudo update-grub" = "failed to get canonical path of /cow" Why?

On Xubuntu 20.04 LTS live USB, I'm trying to add radeon.modeset=0 to the line GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" in the file sudo nano /etc/default/grub so that it reads:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash radeon.modeset=0"

and make that change permanent.

This is to prevent one of my discrete graphic cards (Radeon) from starting on boot.

BUT...

When I sudo update-grub after making the above change, it says failed to get canonical path of /cow...

1. How can I fix that?

2. Is it necessary to sudo update-grub after making the change in order for the change to stick?

3. The change saves successfully and doesn't give a read-only error. However, when I restart, the change is no longer there.

4. I don't want to make the USB persistent.

5. The USB was created with UNetbootin.

6. My main concern is the disabling of the discrete graphic card rather than the title of this question. So if the fix to the title is too involved and the disabling of the graphic card could be accomplished with less work, I'm all for it.

7. https://imgur.com/FCrUBHh

8. Thank you very much for any help! ?