4TB Purple Surveillance Internal Hard Drive HDD from Western Digital From the manufacturer, the WD Purple Surveillance Hard Disk Drive has a 256MB cache 3.5" SATA. 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.