Yes, this is possible. SOME RAID controllers use a special feature of certain HDD's to minimize HDD wear when not actually needed. They take control of the HDD's motor through a special setting on the HDD. When the array has not been used for some specified time, the controller shuts down the HDD's - parks the heads and then turns off the spin motor. When the array is accessed again it takes a few seconds to spin up. Consult the documentation in your RAID5 system to see whether yours is doing this, whether you can cancel it, or how you can adjust the time before shut-down.