Startup processing method for medium storage device, controller for medium storage device, and medium storage device

A medium storage device performs ready processing for enabling a head to access data on a storage medium after power is turned ON, in order to prevent to change the status to offline by the host even if ready time is delayed due to retry. The medium storage device has a rotary storage medium, a head, a control unit, and a memory for storing a specified ready time at which the ready processing is expected to be completed. And the command response of the device is changed to ready status when the specified ready time comes even if the ready processing is not complete within the specified ready time. Therefore when the host issues an access command after the specified ready time elapses, since Ready is responded, the possibility of the host changing the status to offline is eliminated.