Western Digital Purple . HDD capacity: 4 TB HDD speed: 5400 RPM Storage drive buffer size: 256 MB HDD size: 3.5" Interface: Serial ATA III

WD Purple drives are designed to meet the challenges of 24x7 video surveillance recording. These drives are engineered specifically for surveillance to help withstand the elevated heat fluctuations and equipment vibrations within NVR environments. An average desktop drive is built to run for only short intervals not the harsh 24/7 always-on environment of a high-definition surveillance system. With WD Purple drives you get reliable surveillance-class storage thats tested for compatibility in a wide range of security systems. Exclusive AllFrame technology helps reduce frame loss and improve overall video playback.