WD Purple 4TB 3.5-inch Surveillance Hard Drive (WD42PURZ) - Capacity: 4 TB, Interface: SATA, Form Factor: 3.5 Inch, Cache Size: 256MB, Compatibility: WD Purple surveillance storage is built to handle up to 64 cameras per drive and is designed for 24/7, always on, high-definition surveillance security systems with more than eight bays