Oblon, Fisher, Spivak, McClelland & Maier
Information recording disk provided with distinguishing mark
 An information recording disk comprising a disk-like substrate and magnetic layers formed on both surfaces of the substrate is provided with a mark punched through the disk. The mark is provided by, for example, a hole positioned apart from a reference line passing through both the center of an index hole for specifying the circumferential position of the disk and the center of the disk. If the disk is rotated by 180.degree. about the reference line, the position of the mark is reversed with respect to the reference line, making it possible to distinguish the front and back surfaces of the disk. The magnetic layers formed on both surfaces of the substrate by means of, for example, sputtering, differ from each other in magnetic properties. To determine the magnetic layer used for the information recording-reproduction or to determine the optimum conditions for the recording-reproduction, it is necessary to distinguish the front and back surfaces of the disk. The mark provided in the disk of the present invention permits visually distinguish the front and back surfaces of the disk easily in the step of loading the disk in a package. Also, the mark can be provided in the punching step of a tape having magnetic layers formed on both surfaces thereof in a disk-like shape, thereby eliminating confusion in the later step. 
Kabushiki Kaisha Toshiba
G G G
G11B G11B G11B
inventional inventional inventional
Hisashi Tomiya
Ito Sonoda