Thanks for your prompt reply.
After checking the format of the SD card I reinserted the card into the Teensy and retried the Cardinfo example to get the verbose output..
It's embarassing to admit that probably the SD card had not been inserted...
I have run the 'CardInfo' example on my Teensy3.6 and get the message "initialization failed".
I have changed line 40 to read: const int chipSelect = BUILTIN_SDCARD;
I have nothing connected to the Teensy (except the USB cord.) The SD card is...