A control unit, such as an MPU in an external storage device or external storage control device, detects, on the basis of a Read or Write command, whether or not an internal reading or writing velocity is changed from one value to another value according to a zone of a storage medium. When the velocity is changed, the control unit calculates an optimal buffer full ratio or buffer empty ratio suitable for the velocity and sets the ratio in a data buffer control unit. Moreover, the control unit, such as an MPU, determines whether or not data requested by the Read or Write command resides over different zones in the storage medium. When the data resides over different zones, the control unit calculates one zone containing a majority of the requested data, works out an optimal buffer full ratio or buffer empty ratio according to a medium transfer rate associated with the zone, and sets the ratio in the data buffer control unit.