Method for controlling an external storage system having multiple external storage devices

An external storage device is part of a ring-like first route formed by connecting multiple external storage devices to each other via communication channels. The external storage device comprises a first connection unit having a first input port and a first output port, and a second connection unit having a second output port and a second input port that are connected to a first input port and a first output port of an adjacent external storage device on the first route, respectively, to form one communication channel of the first route. When both connection units are enabled, information received by the first input port and the second input port is sent from the second output port and the first output port, respectively. When one of the connection units is disabled, information received by the input port of an enabled connection unit is sent from the output port of the enabled connection unit. If a communication failure has been detected in one communication channel of the first route, connections to the first spare communication channel are enabled, and connections to the failure-detected communication channel are disabled.