Well, some more to report, I think we have a kind of success!

I formatted the SD card in the PC, then placed it in the camera, and without formatting in camera, took some shots.
It seemed to work OK, without in-camera format

Now I have the following method, using the two apps, X-plore and MyFiles

For CF card, format in camera, and access with the tablet using the X-plore app. MyFiles cant see it. Save files to internal SD card.

For SD card, format in PC, and then access with the tablet using the MyFiles app. X-plore cannot see it. Save files to internal SD card.

To transfer from the internal SD card to the USB stick second backup, use Myfiles, X-plore cannot see it.

As long as I can remember all that, I should be OK.

I'm waiting for the arrival of a powered OTG cable, to see if I can access the 250Gb SSD drive.