Starting from Android 4.4.2 they have changed the permissions for external storage.
As it is today with OruxMaps it is not possible to store maps on external storage.
However if OruxMaps could be updated according to the below link it would be possible to store Maps in OruxMaps private app directory on the external storage.
http://developer.android.com/reference/android/Manifest.permission.html#READ_EXTERNAL_STORAGE