Western Digital WD Purple Pro Surveillance HDD 22TB SATA 6 Gb/s 7200 RPM 512 MB Cache: Straight from the manufacturer designed for high-definition security systems that are always on. WD Purple drives are ideal for surveillance systems, supporting up to 64 cameras and a supported workload rate of up to 180 TB/year. Frame load is decreased by AllFrame 4KTM technology.