Android write files to USB via OTG cable

Question

I've been searching for many topics about android file writing, yet most of them wanted to write files to android internal storage. Others who wanted to write files on external SD card didn't success at all. My case is quite similar but I think that writing files to external USB is a totally different case.

I am using Samsung galaxy Note II running stock TouchWiz 4.4.2 [not rooted]. My phone supports micro-USB-OTG and I can mount my USB as rwxrwx--x without rooting. The complete path of my USB is /storage/UsbDriveA.

I've tried to use Environment.getExternalStorageDirectory() to get the path or use the path (mentioned above) directly but neither of them succeed. The first one returns internal storage path and the second one returns an error with "permission denied". I have already put the

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

in Android Manifest so I wondered why my code didn't work.

Moreover, I can write anything to my USB using Root Browser (use it without root) and Simple Browser thus I believe that there's a way to do that.

Here's my code:

File file = new File(path.getAbsolutePath(), "test.txt");
// File file = new File("/storage/extSdCard","test.txt");
err = false;
  try {
    FileOutputStream f = new FileOutputStream(file);
    PrintWriter pw = new PrintWriter(f);
    pw.print(get);
    pw.flush();
    pw.close();
    f.close();
  }catch (Exception e) {
    e.printStackTrace();
    Toast.makeText(MainActivity.this, "writing error",Toast.LENGTH_LONG).show();
                err = true;
   }
   Log.i("File Path:", file.getPath());

Thanks in advance!!!

Answer 1

From android 4.4, you can use Storage Access Framework to access to removable media (see https://commonsware.com/blog/2014/04/09/storage-situation-removable-storage.html). For example, I tried with success to copy a pdf file from local memory to removable memory connected by OTG adapter. The only limitation: the user has to choose a destination folder.

1) call Intent.ACTION_CREATE_DOCUMENT:

Intent intent = new Intent(Intent.ACTION_CREATE_DOCUMENT);
intent.setType("application/pdf");
intent.putExtra(Intent.EXTRA_TITLE, file.getName());
startActivityForResult(intent, REQUEST_CODE);

2) intercept the return intent

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data){
    if(requestCode == REQUEST_CODE) {
        if (resultCode != RESULT_OK) return;
        copyFile(fileToCopy, data.getData());
    }
}

3) use the ContentResolver to open the outputStream and use it to copy the file

private void copyFile(File src, Uri destUri) {
    BufferedInputStream bis = null;
    BufferedOutputStream bos = null;

    try {
        bis = new BufferedInputStream(new FileInputStream(src));
        bos = new BufferedOutputStream(getContentResolver().openOutputStream(destUri));
        byte[] buf = new byte[1024];
        bis.read(buf);
        do {
            bos.write(buf);
        } while(bis.read(buf) != -1);
    } catch (NullPointerException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        try {
            if (bis != null) bis.close();
            if (bos != null) bos.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}