A host bus adaptor driver for installing an operating system in an I/O device unit from a system device. The connection of a single path and a single disk unit to the destination of installation is guaranteed at the time of installing an operating system in the I/O device unit having at least a redundant path or a plurality of disk units. Thus, the user is not required to change the system configuration definition and do the cable insertion/removal job. The host bus adaptor driver includes a device for reading the OS start information held in the host bus adaptors, a device for determining a host bus adaptor and a connection path for installing the operating system based on the OS start information thus read, and a device for permitting the installer of the operating system to use only the host bus adaptor and the connection path thus determined.