When I boot the computer I immediately get the message, "the disk Mac HD laptop - data" hard drive cannot be found." I've looked online and this is a known problem with the update to the latest OS "Big Sur." It goes on to say: "A problem was detected with the disk that prevents it from being unlocked."
It's a known issue you can read about here: https://macreports.com/the-disk-macintosh-hd-data-cant-be-unlocked-fix/
This is my backup computer and I have shortcuts to many subdirectories, so I'm going to take my time, and eventually fix it using these instructions OR, if that doesn't work, will call Apple for help.