I've got four Western Digital 10TB WD Purple Pro Surveillance Internal Hard Drive HDD - SATA 6 Gb/s, 256 MB Cache, 3.5" - WD101PURP in my Empire Tech NVR. Covers me for 21 days using 15 cameras. I'm going to be brave and up the TB and see what happens should any of them fail before I upgrade the NVR.