According to a copy instruction from a host, a session in which entire or partial data of a primary volume of a first RAID device of the copy source is transferred and copied to the same position of a secondary volume of a second RAID device of the copy destination through an inter-chassis path is established and executed based on session information. A computer of the host instructs the first RAID device of forcible stop of the session as to cause the session information to be deleted when the inter-chassis path is blocked, and causes the session information remaining in the second RAID device to be deleted by an instruction given via the first RAID device when the forcibly terminated session is tried to be started when the blocked inter-device path is connected and results in an error.