The security cam Python file can be easily adapted to save the repeated snapshots of detected people by modifying line 235 to something like the following, where my USB drive is called 'KINSTON':

photo = ( "/media/pi/KINGSTON" + "/captures/photo_" + cur_time + ".jpg" )

In reality, using this USB stick slowed down the program rather drastically! Images can be transferred / deleted easily from the Pi if another PC is used to SSH into the Pi once it's deployed.