In a magnetic or optical disk drive the integrity of data written on a disk is verified by writing data to a portion of the disk, and immediately reading the data from the disk after it has been written. The dielectric layer 14 has a site containing particles of a dielectric material which is formed of only one of these particles in its thickness direction. When the mirror holder is at a mirror-up position, the follower portion traces a second region of the cam portion to urge the mirror holder toward the mirror-up position by the urging force of the urging member. Each tooth has a straight, relatively long, base section and a straight, relatively short, cutting section bent from one end of the base section toward the inner surface to form an obtuse angle between the inner surface of the base section and the inner surface of the cutting section.