Your error:
SD errorCode: 0X1
SD errorData: 0XFF
Means that your card did not accept the first command, CMD0, and timed out after two seconds. This means SdFat does not see the card on the SPI bus.
I don't think the SPI pins are connected to shield pins 11, 12, and 13. They are connected to the ICSP header so the Ethernet shield will work on either a 328 or Mega.
I don't know what MarkT means by
driving the SPI bus too slow and not releasing MISO
SD cards are initialized at a slow SPI speed and after initialization the spec allows a wide range of speeds.
On the Arduino, the state of MISO is controlled by the SPI hardware and the devices/shields connected to MISO. There are no software options to control MISO. MISO is an input.