docs: def drive_rotational_speed_rpm(self):
          """Gets set of rotational speed of the disks"""
  
          drv_rot_speed_rpm = set()
          for member in self._drives_list():
              if member.rotation_speed_rpm is not None:
                  drv_rot_speed_rpm.add(member.rotation_speed_rpm)
          return drv_rot_speed_rpm