Comment 7 for bug 1901137

As additional confirmation that the installer was trying to write the MBR to the install media, I was successful in installing 20.10 from the same install media on the same hardware by booting the install media to ram, as described here: https://askubuntu.com/questions/829917/can-i-boot-a-live-usb-fully-to-ram-allowing-me-to-remove-the-disk.

I then removed the install media once live, and ran the install, and all installation went without a hitch, and I am now commenting from a fresh, new 20.10 install.

In my case, install media is a removable SD card, if that's pertinent.