In investigating the cause of a fault in a computer storage system, it is considered useful to previously prepare maintenance logical units (LUs) of a simple structure, the operation of which has been confirmed. If the same number of LUs as servers are prepared for each server as in the prior art, the efficiency is low. Furthermore, securing these LUs complicates assignment of the LUs for construction of a system and a work for addressing the fault. The present invention provides a computer system free of these problems. The computer system has a first computer for executing a first OS (operating system), a second computer for executing a second OS, and a storage array system. The storage array system uses a disk device having a logical unit (LU) for storing a boot loader, as well as the first and second OSes. The boot loader is executed on any one of the two computers, reads in any of the OSes corresponding to the currently operating computer into this operating computer, and executes the read OS.