Arduino/libraries/SD/examples/ReadWrite/ReadWrite.pde

#include <SD.h>

File f;

void setup()
{
  Serial.begin(9600);
  Serial.print("Initializing SD card...");
  SD.begin();
  Serial.println("done.");
  
  f = SD.open("test.txt", true, false);
  if (f) {
    Serial.print("Writing to test.txt...");
    f.println("testing 1, 2, 3.");
    f.close();
    Serial.println("done.");
  } else {
    Serial.println("error opening test.txt");
  }
  
  f = SD.open("test.txt");
  if (f) {
    Serial.println("test.txt:");
    while (f.available()) Serial.write(f.read());
    f.close();
  } else {
    Serial.println("error opening test.txt");
  }
}

void loop()
{
}