Big Sur Upgrade stalls saying “This disk is used for Time Machine backups”

Hi! I’m trying to upgrade a Mac mini running High Sierra to Big Sur (or any newer OS). Upon downloading and running the Big Sur upgrade, I get to the point where I select the disk to install to and it won’t let me choose the Macintosh HD because of a pop up that reads, “This disk is used for Time Machine backups.” So I can’t select any drive to install to. I do use Time Machine, but, obviously, it backs up “Macintosh HD” to a different drive. I’ve reviewed the Time Machine Settings and nothing refers back to Macintosh HD as a destination drive.

The same results are reached when I try, instead, to upgrade to Mojave or Catalina. I have also created a bootable USB drive for Mojave installation, which works on other Macs, but on the Mac Mini in question will not even start the installer, but rather bumps to rescue mode (even though alt/option is pressed, not command-r).

I’m stumped. Thanks for any thoughts.