Have spent quite a few hours today trying to install Big Sur on to an external SSD in a last-ditch effort to make a late-2014 Mac mini usable. (From what I understand the drive is what makes it painfully slow.) I’ve successfully installed to the SSD but am not able to select it as a alternate startup option. I think I need to enable booting from external drives, which you apparently do from recovery mode…only my mini doesn’t boot into recovery mode at all (even though I can see a partition that looks like it should be one). So now I think I need to make a bootable install USB to try and somehow get into recovery mode, to enable the external disk, to run os x from the external drive, so I won’t go bananas trying to use an old computer. Whew.