Problem found. I tried formatting a card with the SdFat formatter . A card formatted thus, works fine with SdFat. I had been using GParted to format cards. SD had no trouble with them on the Teensy4.0. SdFat also worked fine on the nano. but apparently SdFat runnig on T4 expects a possibly better installation. Now I have to go back and undo the probably stupid things I did to my application while I was puzzling this out.

a little strange, don't you think?