In my previous posts I neglected to mention that I always start with a freshly formatted card, for every device, in FAT32 using SD Formatter. On my last attempt, when powering on (with duplicate card installed) the processing cycle completed but the books did not appear -- yet were accounted for in Calibre. The card is an exact duplicate of the one installed, just a different capacity.
I've been dragging my feet but today I plan to start from scratch and resend each book. I am still scratching my head, however.