A disk drive and an information processing system optimizes the number of rotations of a disk. Data read from a disk and previously-read cache data are temporarily stored in buffer memory. When a total amount of cache data achieved during a given period of time T1 or an average amount of the same is equal to or less than a threshold value X1, a system controller controls the number of rotations in an increasing manner, thereby optimizing the number of rotations. When a total amount of cache data achieved during a given period of time T2 (T1