Permission denied when creating new file on external storage

I need to copy a file from the assets to the external storage. Here is my code:

    File f = new File(Environment.getExternalStorageDirectory() + File.separator
            + "MyApp" + File.separator + "tessdata" + File.separator + "eng.traineddata");
    if (!f.exists()) {
    AssetManager assetManager = getAssets();
    try {
        f.createNewFile();
        InputStream in = assetManager.open("eng.traineddata");
        OutputStream out = new FileOutputStream(Environment.getExternalStorageDirectory() + File.separator
            + "MyApp" + File.separator  + "tessdata" + File.separator + "eng.traineddata");
        byte[] buffer = new byte[1024];
        int read;
            while ((read = in.read(buffer)) != -1)
                    out.write(buffer, 0, read);
            in.close();
            in = null;
            out.flush();
            out.close();
            out = null;
        } catch (IOException e) {
            Log.e("tag", "Failed to copy asset file: ", e);
        }
    }

I've also added the permission in the manifet

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

The exception occurred when is executed f.createNewFile(), saying "Permission denied".

How can I fix it please?