Thanks @rickkas7 for your prompt help.
While I wait on this to be resolved, the remaining insurmountable problem (for me) is that I cannot use the Ethernet Featherwing and an SD Feather card together. The code locks up. I tried to disable the Ethernet while I write to the SD card then re-enable it after that but this solution is not reliable.
Is there any workaround?
Thanks in advance for your help.