A storage system and data management method is provided that improves the reliability and fault tolerance of the hard disks saving data utilizing an AOU function. A storage system comprises a first correlating section for correlating a plurality of RAID groups composed of a plurality of physical disks, and the pool region, a second correlating section for correlating the pool region and the storage regions of the virtual volumes, a first allocation section for allocating first data from the host apparatus to the first storage region of the first RAID group based on write requests from the host apparatus, and a second allocation section for distributing second data from the host apparatus at and allocating the second data to any storage regions of the RAID group, with the exception of the first storage region of the first RAID group allocated by the first allocation section, based on write requests.