We've recently been struggling with a hardware issue concerning activating the SD card reliably on the Uno.
We have it set to all of the stock pins, with the exception of chip select, which is on Pin 10 instead of Pin 4. We've had it working several times and successfully written data, however, now, no matter the configuration we put the card in, and even using the stock SD card test moving back to pin 4, we cannot get the card to function reliably with the Uno. However, it functions normally with an SD card slot on a PC.
I've attached the two most common pin setups we used, a basic generic direct mapping from pin to pin with a 3.3v Supply off the board to V_DD, or our voltage divided supply so that all pins are operating on 3.3V.