A disk array system, coupled to a host computer, comprising: a disk unit having a plurality of hard disk drives capable of changing operating modes based on settings and at least one expander device each including a memory and a plurality of ports each coupled to one of the plurality of hard disk drives; and a disk controller being coupled to the disk unit and including a processor (124a, 124b) for controlling data transfer between the host computer and the plurality of hard disk drives via the at least one expander device, characterized in that the at least one expander device manages a particular parameter value for its each port coupled to one of the plurality of the hard disk drives on the memory thereof and optimizes the status of the data transfer between the host computer and each of the plurality of the hard disk drives based on the particular parameter value for each port thereof, each of the plurality of hard disk drives stores their own particular parameter information regarding a parameter value suited for a high speed communication mode wherein the hard disk drive communicates with the expander device at 6 GBps; the processor (124a, 124b) is configured to: control, as triggered by attachment of a first hard disk drive of the plurality of hard disk drives, the first hard disk drive and a first expander device including a first port coupled to the first hard disk drive to link up in a low speed communication mode wherein the first hard disk drive communicates with the first expander device at 3 GBps; read the particular parameter information from the first hard disk drive in the low speed communication mode; update the particular parameter value for the first port, which corresponds to the first hard disk drive, managed on the memory of the first expander device; and control the first hard disk drive and the first expander device to link up in the high speed communication mode wherein the first hard disk drive communicates with the first expander device at 6 GBps. a disk unit having a plurality of hard disk drives capable of changing operating modes based on settings and at least one expander device each including a memory and a plurality of ports each coupled to one of the plurality of hard disk drives; and a disk controller being coupled to the disk unit and including a processor (124a, 124b) for controlling data transfer between the host computer and the plurality of hard disk drives via the at least one expander device, characterized in that the at least one expander device manages a particular parameter value for its each port coupled to one of the plurality of the hard disk drives on the memory thereof and optimizes the status of the data transfer between the host computer and each of the plurality of the hard disk drives based on the particular parameter value for each port thereof, each of the plurality of hard disk drives stores their own particular parameter information regarding a parameter value suited for a high speed communication mode wherein the hard disk drive communicates with the expander device at 6 GBps; the processor (124a, 124b) is configured to: control, as triggered by attachment of a first hard disk drive of the plurality of hard disk drives, the first hard disk drive and a first expander device including a first port coupled to the first hard disk drive to link up in a low speed communication mode wherein the first hard disk drive communicates with the first expander device at 3 GBps; read the particular parameter information from the first hard disk drive in the low speed communication mode; update the particular parameter value for the first port, which corresponds to the first hard disk drive, managed on the memory of the first expander device; and control the first hard disk drive and the first expander device to link up in the high speed communication mode wherein the first hard disk drive communicates with the first expander device at 6 GBps. control, as triggered by attachment of a first hard disk drive of the plurality of hard disk drives, the first hard disk drive and a first expander device including a first port coupled to the first hard disk drive to link up in a low speed communication mode wherein the first hard disk drive communicates with the first expander device at 3 GBps; read the particular parameter information from the first hard disk drive in the low speed communication mode; update the particular parameter value for the first port, which corresponds to the first hard disk drive, managed on the memory of the first expander device; and control the first hard disk drive and the first expander device to link up in the high speed communication mode wherein the first hard disk drive communicates with the first expander device at 6 GBps.