SD file.available() not working

Hi, does anyone can use the SD bridge function file.available()? If I try to run the datalogger example (http://arduino.cc/en/Tutorial/YunDatalogger) adding a check for the SD to be written or not:

....
  // The FileSystem card is mounted at the following "/mnt/FileSystema1"
  File dataFile = FileSystem.open("/mnt/sd/datalog.txt", FILE_APPEND);

  if(dataFile.available()>0)
    Serial.println("file already written");

  // if the file is available, write to it:
  if (dataFile) {
    dataFile.println(dataString);
    dataFile.close();
...

The program always stop on file available. looking at FileIO.cpp does not help so much. Any suggestion? Thanks in advance