int CS_PIN = 4;

File file;

void setup()
{

  Serial.begin(9600);

  pinMode(10, OUTPUT);
  digitalWrite(10, HIGH);

  Serial.print("Initializing SD card...");

  if (!SD.begin(CS_PIN)) {
    Serial.println("Card failed, or not present");
    return;
  }
  Serial.println("card initialized.");


}

void loop() {

  String dataString = "TEST";

  File dataFile = SD.open("datalog.txt");

  if (dataFile) {
    dataFile.println(dataString);
    dataFile.close();
    Serial.println(dataString);
  } else {
    Serial.println("error opening datalog.txt");
  }

  delay(1000);

}

SD.open("file.txt", O_RDWR | O_APPEND);

SD.open("file.txt", O_RDWR | O_CREAT);