Western Digital WD4003FFBX Hard Drive, Capacity 4TB Red PRO, 3.5" SATA3, NAS Hard Drive, 4TB, 256MB, Form Factor 3.5", SATA 6GB/s, RPM 7,200. Designed specifically with medium or large scale business customers in mind, WD Red Pro drives are available for up to 24-bay NAS systems. Engineered to handle high-intensity workloads1 in 24x7 environments.