If you download the osmdroid: osmdroid-android-3.0.8-sources code package, you can open the OpenStreetMapTileProviderConstants.java class and change the variable like this:
Modify
public static final File OSMDROID_PATH = new File("/mnt/sdcard/osmdroid");
to*
public static final File OSMDROID_PATH = new File("/mnt/ext_sdcard/yourfile");
Then put your map tiles in yourfile .
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