A storage system is composed of a first storage subsystem and a second storage subsystem each having one or more volumes, and a third storage subsystem capable of being accessed from both the first storage subsystem and the second storage subsystem. The storage system operates to replicate the data written from the host to a volume within the first or the second storage subsystem to a volume within the second or the first storage subsystem. The first and second storage subsystems periodically write health check information to the third storage subsystem. When the first storage subsystem receives a write request from the host but cannot replicate the write data to the second storage subsystem, it reads the health check information written to the third storage subsystem to confirm that a volume of the second storage subsystem is I/O prohibited, and resumes processing regarding the write request from the host.