A method according to one embodiment includes acquiring a result of quality inspection related to recording quality of first sectors included in a first storage area of a magnetic disk. The method includes calculating a first number being the number of sectors corresponding to a capacity obtained by subtracting a set capacity from an actual capacity of the first storage area. The first number is calculated based on the set capacity of the first storage area and a set value of recording density of the first storage area. The method includes selecting a second number of sectors from the first sectors based on the result of quality inspection. The method includes assigning addresses to a group of sectors that are not included in the second number of sectors among the first sectors. The group of sectors consists of sectors with the number corresponding to the set capacity.

According to an embodiment, tracks on a magnetic disk each include a long-distance sector having a length in the circumferential direction covering two or more servo sectors. A controller executes an acquisition operation to acquire one or more evaluation amounts on the basis of a track pitch in each of the two or more servo sectors included in a portion adjacent to the long-distance sector. The controller executes a protection operation to protect data of an adjacent track in a case where a total value of the one or more evaluation amounts exceeds a first threshold value.