Felix, thanks for UF2 you emailed. When I load that UF2 via USB drive the LED blinks slowly to indicate the flash was detected. But when I compile and load your test sketch using Arduino IDE I get the not-detected result I wrote about above. So it seems something is amiss with my libraries, but I can't think what it would be. I just installed SPIFlash, and the Moteino SAMD M0 board in the board manager, so these should be up-to-date.