1. A storage apparatus that provides to a host computer a logical device for storing data sent from said host computer, comprising: a nonvolatile memory for storing said data; a Hard Disk Device (HDD) for storing said data; and a controller for controlling said nonvolatile memory and said HDD; wherein said controller configures said logical device with a first logical unit corresponding to said nonvolatile memory and a second logical unit corresponding to said HDD, in which said first logical unit is controlled by mirroring to said second logical unit, wherein said controller controls to manage power on and off modes of said HDD and an operating time of said HDD in said power on mode, and wherein if said controller receives an access request to said logical device from said host computer, said controller controls to read/write data from/to one of said first and the second logical units based on said power on and off modes of said HDD.