Why does my SD card corrupt on arduino rescue?

I have a 4GB Kingston micro SD card and a datalogger code that deletes the exiting file and creates a new one with the new values of the variables. Below is the Code:

void back_var() {

  SD.remove("/backup/BACKUP.txt");
  char var_up[] = "%d;%d;%d;%d;%d;%d;%d;%d;%d;%d;%d;%d;%d;%d;%d;%d;";
  sprintf(
    variaveis,        //Nome da String a ser criada
    var_up,           //Char utilizado para armazenar a String
    valor1,    //Informações
    valor2,
    valor3,
    valor4,
    valor5,
    valor6,
    valor7,
    valor8,
    valor9,
    valor10,
    valor11,
    valor11,
    valor12,
    valor13,
    valor14,
    valor15
  );
  Serial.println("Print das variáveis:");
  Serial.println(variaveis);
  Serial.println(botoeira_entrada_S15);
  SD.begin (4);
  backup = SD.open("/backup/BACKUP.txt", FILE_WRITE);
  if (backup) {
    Serial.println("Inserindo variáveis no SD...");
    backup.print(variaveis);
    backup.print('|');
    Serial.println("Variáveis inseridas no SD!");
    backup.close();



  }
  else {
    Serial.println("Erro ao abrir arquivo de Update");
  }
}

The problem is that after a certain time the txt file is corrupted and other unreadable files are created. Why does this occur? And how to solve? Below are the unreadable files created: