When the application is started (with the external sdcard W/R permission enabled), the 'SLAM' folder (moved to '/sdcard/Download/SLAM') is not founded and the SLAM system is not correctly initialized.
It appears that the sdcard drive (with its vocabulary and calibration info) is not accessible immediately (when the application starts). The suggested changes is a quick fix, the data retrieval should ideally be handled in a separate function called during the initialization phase (from 'OrbTest.java').
Note: On the tested device, EXTERNAL_STORAGE variable is set to '/sdcard'.