The best source of recent information is the Arduino forum

http://arduino.cc/forum/

Also search the Adafruit forum

http://forums.adafruit.com/

If you are using a Teensy try

http://forum.pjrc.com/forum.php

Try running the SdInfo sketch in the SdFat/examples folder to get 
more information.


The most common problems are:

--------------------------------------------------------------------------------
Wiring errors with card.init() failure and errorCode == 1.  This means the
first command, CMD0 - SPI init, failed.  SdFat is not seeing the card.
This is a basic hardware problem.  Do not reformat your SD.
--------------------------------------------------------------------------------
card.init() failures caused by resistor SPI level shifters. Some, mostly older,
SD cards work with resistor level shifters but may be flaky with random errors.

Many newer SD cards have edge detectors and fail with errorCode() = 1.  These
cards will not work with resistor level shifters since SdFat does not see the
card at low SPI clock rate.

Other cards fail with an errorCode() > 1.  These cards see SD commands at low
SPI clock rate but fail at higher SPI clock rate.  It may be possible to use
these cards by replacing card.init() with card.init(SPI_HALF_SPEED).  
This will slow the the SPI clock by 50%.

--------------------------------------------------------------------------------
Redefining SS_PIN to be a pin other than the Arduino/AVR hardware SS pin.

Arduino/AVR SPI will only function correctly in master mode if the hardware SS
pin is in output mode. Sd2Card::init() will enable output mode for the hardware
SS pin even if it is not SD chip select.

If you have multiple SPI devices you must insure devices other than the SD card
have their chip select pin high when Sd2Card::init() is called.

If you want to understand in more detail how AVR SPI works read this

http://www.atmel.com/dyn/resources/prod_documents/doc2585.pdf. See section 2.3.