A RAID system includes a pair of RAID controllers adapted to operate in active-active mode, each controller including a cache memory and at least one SAS/SATA I/O chip connected to a plurality of hard disk drives. Each SAS/SATA I/O chip includes more SAS/SATA ports than required to carry data to the hard drives. The caches in the respective controllers are synchronized via the extra SAS/SATA ports in each controller.