unrecognizible SSD startup drive

Thanks for the explanation, I wasn't aware of the disk restructure. I assume there is an advantage to the APFS format. This brings up another problem though, my mail and browsers in Mojave consider I'm on a new machine. Browsers are missing bookmarks, mail has no addresses, applications are asking for license fees, etc. I am probably missing something about syncing the old to the new.

Eric Roots suggestion for switching using the option key on restart works great, so I can go back and forth, at least, but you'd think they would have a built-in migration ability.

Thank you, Eric, that's just what I needed. Do you have any suggestions for how to easily migrate my bookmarks, addresses, contacts and paid app licenses to my Mojave OS?