This may a false alarm. In the test described, I had the DFU switch on and was using the default connections between Notecard and host.
I turned DFU off, used the connections described in https://discuss.blues.com/t/esp32-s2-outboard-dfu/1125 and it is now writing the new version to the host.
Still debugging, the host is not successfully updating, but I made it past this hurdle.
Karl