In response to read and write requests for data blocks at logical storage locations, a block mapping device determines physical storage locations on the storage devices. Read requests switch over secondary storage devices to the active mode when they are in the passive mode. Write requests write data blocks only to the primary storage devices. Secondary storage devices that have not been accessed for a minimum activation time may be switched over from the active to the passive mode to save power consumption and cooling. Data migration and data recall policies control moving of data blocks between the primary and secondary storage devices and are primarily based on threshold values.