That last info appears to have done the trick. . . The part about the earlier OS not being aware of the existence of APFS was not lodged firmly in my skullbank. It's a New Era Mac-wise and I need to update my knowledge accordingly.
Many thanks for your help; I didn't expect a response on July 4th. But you have always had the speediest customer support that I have experienced in 24 years using Macs, so I guess that I shouldn't have been surprised.
