Here is the breakout:https://learn.adafruit.com/adafruit-micro-sd-breakout-board-card-tutorial
I have tested with several cards, including a SanDisk Extreme Plus - doesn't seem to be a factor in this instance.
This is the one of the discussions that sent me down the buffer path - https://forum.arduino.cc/t/explanation-of-sd-write-and-flush-please/369320
Of course one of the contributors is the authority on the subject, and seems to give a differing answer to the others...
Thank you all for such a great response, it is really appreciated!