This invention relates to the emulation processing method of a storage device and the storage device for accessing storage medium from a host that has a different sector size, and where alternate processing is performed in the sector unit of storage medium even when there is a writing error when updating the disk. In a system where a second sector size of a host is smaller than a first sector size of the storage medium, data that are staged in a buffer according to a request from the host is rewritten using write data, and then staged sector data or the rewritten sector data are saved in an alternate area and the storage medium is updated with the rewritten sector data. Since the sector data before updating is saved beforehand, it is possible to perform alternate processing using the sector unit of the storage medium.