This is the SD Card library, formerly known as 'SD'.

To use this library to support SD Cards on the ATXMega, you merely need to

    #include <XSD.h>

in your application in lieu of 'SD.h'. So long as your '~/sketchbook/libraries' directory contains the 'XSD' sub-directory tree, you should be able to make use of it exactly as the standard SD library from the Arduino IDE.

NOTE: this code is from the 1.05 version of Arduino. It should be forward compatible, but may not be backward compatible.