I'm confused with Android emulators. Either emulator for Android KitKat or Lollipop doesn't emulate external disk writing properly.
Both of the Android versions are supposed to prevent applications to make modification on external disk (writing or modifying), even though the app has defined
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
I tried in Android official emulator and also in Genymotion.
Best How To :
When you create a new AVD and set up an SD Card, you add a primary external storage to the emulator. Android from version 4.4 prevents the modification of secondary external storage as you can see here:
The WRITE_EXTERNAL_STORAGE permission must only grant write access to the primary external storage on a device. Apps must not be allowed to write to secondary external storage devices, except in their package-specific directories as allowed by synthesized permissions. Restricting writes in this way ensures the system can clean up files when applications are uninstalled.
For this reason your app can write to the external storage in the emulator.
Unfortunately the default emulator cannot emulate secondary external storage.