Java android Simple XML save file

When I parse java object to XML. I did this :

Serializer serializer = new Persister();
String path = Environment.getExternalStorageDirectory() +
    File.separator + "yourFolder";

File folder = new File(path);
File file1 = new File(folder, "newsnew.xml");
try {
    serializer.write(LFRFIDActivity.root, file1);
} catch (Exception e) {
    // TODO Auto-generated catch block

And I have a problem when I connect a device with my pc doesn’t see a file. But when I restart a deice and connect with my pc my device see a file. What I do wrong ? My XML file is very large it has 11k lines