FileOutputStream fout = null; 
ObjectOutputStream oos = null;
File dxt=Environment.getExternalStorageDirectory();
fout = context.openFileOutput(dxt + "setting.dat",Context.MODE_APPEND);


or
File dxt=new File(Environment.getExternalStorageDirectory(), "setting.dat");
File dxt=new File(Environment.getExternalStorageDirectory() + "setting.dat");
fout = new FileOutputStream(dxt, true);

이런식으로 해도 안되네요..

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission> 퍼미션도 줬는데..