The "fix" for me is log out the existing user and then log right back in, same user, no changes. The drives appear in finder and disk util and work as expected. I spent an hour+ with Apple support and their answers were to "repair the drive", "run first aid", and "reinstall the OS". Starting in recovery mode, the drives show up and are mounted. As soon as I restart normally, the same issue...drives are unable to be mounted. I am not sure why the process of logging out and back in and booting into recovery mode can't illuminate what might be the issue. Log out and back in...drives visible and usable. But every time you disconnect the drives you have to do the same thing in order to mount them.