No; in fact, that is the right way. No mechanical HDD (one with spinning disks and moving heads) can move data as fast as the 3.0 Gb/s "speed rating". That rating is the max speed for the COMMUNICATION between HDD unit and SATA port on the mobo. But the spinning disks are slower that that. Since your RAID5 system will have each HDD on its own SATA 3.0 Gb/s port, those ports are fully capable of handling the data rates involved. It would actually be a waste to put any mechanical HDD on a SATA 6.0 Gb/s port.