Here is a simple test aia, that saves a text file in the ASD on all Android versions:
ReadWriteToASD_extVer2.aia (8.6 KB)

Note:
READ/WRITEpermissions are only required on Android < 4.4 (API 19, KitKat). So I used version 2 of my extension. It's sufficient to ask forWRITEpermission because it implicitily grants READ permission as wll.
