Android 6: Can't save text files to external sd card
To write to an external drive you need to add the necessary permissions to your manifest. To write you will want to add: 

&lt;uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /&gt; 

To read from the drive you should add: 

&lt;uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" /&gt;

You should be able to get away without the read permission but if you want to futureproof your app and make it so users don't run into bugs add it in. Google's API explains the proper guidelines for handling datafiles here.