code: def start(self):
          
          if self._status == TransferState.PREPARING:
              self._running.set()
              super(Download, self).start()
              self._status = TransferState.RUNNING
              self._time_started = time.time()
          else:
              raise SbgError(
                  'Unable to start. Download not in PREPARING state.'
              )
docs: Starts the download.
          :raises SbgError: If download is not in PREPARING state.