when-ever i delete something off of the mmc while connected to mac and unplug it, i do not see those files listed anywhere inside mmc not even hidden but the space were gone what i did was once i dragged and dropped the files from mmc to trash i cleaned the trash and that solved it. i got my lost space back, i do not use ubuntu but you might wanna plug your phone and connect through massstorage and clean your trash(recycle) bin.