Arduino writes only one time on SD card - Data log

Question

I connected the SD card to Arduino as it's explained here. The problem is that the Arduino writes only once to the SD card and I don't understand the reason.

#include <SD.h>
#include <SPI.h>

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);
}

The output on Serial Monitor:

Arduino write only one time on SD card error opening datalog.txt error opening datalog.txt error opening datalog.txt error opening datalog.txt error opening datalog.txt error opening datalog.txt error opening datalog.txt error opening datalog.txt error opening datalog.txt error opening datalog.txt error opening datalog.txt error opening datalog.txt error opening datalog.txt

I also tried to use:

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

As I saw Adding data to an SD card file, but it doesn't work.

Answer 1

You are opening the file in read mode but should be opening it in write mode. Change File dataFile = SD.open("datalog.txt"); to File dataFile = SD.open("datalog.txt", FILE_WRITE);.

You can remove this line of code: digitalWrite(10, HIGH);.

The arduino.cc website is also an excellent source of information regarding the SD library.

Answer 2

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

Repeatedly opening the same file is probably not a good practice, if you want to retain previously written contents.