WD | Purple 4TB Surveillance Hard Disk Drive - 5400 RPM Class SATA 6 Gb/s 64MB Cache 3.5 Inch | WD40PURZIdeal for Built for personal WD Purple Surveillance Hard Drives are engineered for 24 7 reliable operation in HD security systems with up to eight hard drives and up to 32 cameras. SINGLE PACK Engineered specifically for surveillance security systems Reduced video frame loss Tuned for write intensive, low bit rate, high stream count applications typical to most surveillance applications Prioritized write operations for maximized surveillance