For IP security camera systems that record in HD or 4K, it is recommened to have a minimum of 1TB of hard drive in the NVR for every 4 cameras. Hard drives can be added to any NVR if you would like to be able to go further back in time when playing back recorded footage. Typically a 4 camera system with 1TB will be able to record 2-6 weeks of footage depending on whether recording is set to motion-only or 24/7 recording.