#include <SD.h>
#define CS_PIN 10
File file;

void setup() {
  Serial.begin(9600);

  if ( SD.begin(CS_PIN) ) {
    Serial.println("Card initialization OK");
  }
 
  file = SD.open("myfile.txt");
  if ( file ) {
    Serial.println("File is opened!");
    Serial.print("It`s text from file ");
    Serial.print( file.name() );
    Serial.print(" with size ");
    Serial.print( file.size() );
    Serial.println(" bytes ");

    while ( file.available() ) {
      byte b = file.read();
      Serial.write( b );
    }

    file.close(); // закрити файл
  }

  Serial.println("======== TEST WRITE TO FILE ====== ");

  if ( file = SD.open("myfile.txt", FILE_WRITE ) ) {
    Serial.println("File opened for writing!");
    file.println("My text ");
    for (int i=0; i<10; i++) {
      file.print( i );
      file.print( "," );
    }
    file.close();

  }

    file = SD.open("file2.txt");
    while ( file.available() ) {
      byte b = file.read();
      Serial.write( b );
    }
    file.close();

}

void loop() {
  // put your main code here, to run repeatedly:

}