When a medium casing is properly inserted, a first pair of projections and a second pair of projections are passed through between a first bottom surface and a second bottom surface of a chassis in the moving direction and are thus prevented from coming into contact with the first and second bottom surfaces, thereby allowing for smooth insertion of the medium casing. The lower rack includes at least one rack roller mechanism on each laterally-opposed side portion of the rack member, for facilitating movement of the rack member with respect to the lower end of the tub and the interior portion of the open door. In another aspect, a data-driven bandwidth selection technique is provided for the variable bandwidth mean shift method, which estimates for each data point the covariance matrix that is the most stable across a plurality of scales.