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Boa tarde galera, estou com um pequeno problema para escrever no SD card usando o arduino mega+Ethernet Shield, eu consigo abrir o arquivo e escrever nele uma vez, mas na segunda vez ele da erro, segue o codigo, abraços.

Code:

#include <glcd.h>
#include <fonts/allFonts.h>
#include <Keypad.h>
#include <SD.h>

//DECLARAÇÃO DO TECLADO
unsigned long startMillis;
unsigned int iter = 0;
unsigned int iterStart = 0;


const byte ROWS = 4; //four rows
const byte COLS = 4; //three columns

char keys[ROWS][COLS] = {
  {
    '1','2','3','A'          }
  ,
  {
    '4','5','6','B'          }
  ,
  {
    '7','8','9','C'          }
  ,
  {
    '*','0','#','D'          }
};

byte rowPins[ROWS] = {
  A12, A13, A14, A15}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {
  A8, A9, A10,A11}; //connect to the column pinouts of the keypad

Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );
char key = keypad.getKey();

char id[5];
int a=0;


const int chipSelect =10;


void setup() {
  Serial.begin(9600);
  pinMode(10, OUTPUT);

  //INICIALIZAÇÃO DO DISPLAY
  GLCD.Init(NON_INVERTED);   // initialise the library
  GLCD.ClearScreen();        //limpa lcd
  GLCD.SelectFont(System5x7);       // select fixed width system font
  GLCD.SetTextMode(SCROLL_UP);

}

void loop(){
  int i=0;
  delay(150);
  GLCD.ClearScreen();

  GLCD.CursorTo(0,0);
  GLCD.print("ID=");

  //SE NÃO FOI DIGITADO NENHUM ID AINDA ID=0000   
  if (a == 0){ //A VARIAVEL FOR FALSA ELE JOGA O ID = 0000
    for(i=0;i<5;i++){
      id[i]=0;
    }
  }   

  while (i < 5 ){   //MOSTRA O ID                             
    GLCD.CursorTo(i+4,0);   
    GLCD.print(id[i]);
    i++;
  }
  GLCD.CursorTo(0,1);
  GLCD.println("PRECIONE A FUNCAO    DESEJADA");
  GLCD.println("A=Identificação");
  GLCD.println("B=Mensagens Diversas");

  //MENU DE ESCOLHA
  switch (key = keypad.getKey()){

 case 'B':
    funcoes();
    break; 


  }
}


//**************--------------****************************************---------------------------------------**********

void funcoes(){
   File myFile;

  if (!SD.begin(4)) {
    Serial.println("initialization failed!");
    return;
  }
  Serial.println("initialization done.");

  // open the file. note that only one file can be open at a time,
  // so you have to close this one before opening another.
  myFile = SD.open("test.txt", FILE_WRITE);

  // if the file opened okay, write to it:
  if (myFile) {
    Serial.print("Writing to test.txt...");
    myFile.println("testing 1, 2, 3.");
    // close the file:
    myFile.close();
    Serial.println("done.");
  }
  else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");
  }

  // re-open the file for reading:
  myFile = SD.open("test.txt");
  if (myFile) {
    Serial.println("test.txt:");

    // read from the file until there's nothing else in it:
    while (myFile.available()) {
      Serial.write(myFile.read());
    }
    // close the file:
    myFile.close();
   
  }
  else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");
  }
 
  delay(2000);
}