SD.begin() returns false

PaulS:
I wouldn't assume, then, that 10 is the correct pin.

Im sorry. I thought that the chipselect pin is necesary for all communication via SPI.

I have an Arduino Uno R3 with an ATMEGA 328.

My wiring is this:
10 - CS
11 - MOSI
12 - MISO
13 - SCLK

I tried all other pins (2 - 9) for the CS pin as well.