32 Channel 8 HDD 2U 4K NVR

Use surveillance-grade disks, or at least NAS-class disks rated for 24/7 operation. Desktop disks are commonly specified for about 2,400 power-on hours per year (24/7 operation is 8,760 hours) and a workload of around 55 TB/year, while surveillance disks are designed for continuous operation and typically rated at 180 TB/year, with higher-end surveillance and AI models often rated at up to about 550 TB/year. Surveillance firmware is optimized for many simultaneous write streams, typically up to 64 HD cameras per disk, and keeps the stream flowing instead of retrying a weak sector for a long time, which helps prevent dropped frames; many larger models also have vibration sensors for multi-bay chassis. A desktop disk may work at first, but under 24/7 recording it usually develops gaps, errors and a shorter life.

The workload rating is the amount of data the manufacturer expects the disk to read plus write per year while still meeting its reliability specification; typical values are 55 TB/year for desktop disks and 180 TB/year for surveillance disks. One megabit per second of continuous traffic equals about 3.94 TB per year, so a 180 TB/year disk corresponds to roughly 45 Mbps of round-the-clock traffic, including playback and backup reads. Example: 16 cameras at 4 Mbps recorded on a single disk means 64 Mbps, or about 252 TB/year, which exceeds a 180 TB/year rating; spreading the recording over two disks (about 126 TB/year each) or choosing a higher-rated model solves this. Exceeding the rating does not stop the disk immediately, but it increases the probability of failure.

CMR (conventional magnetic recording) writes tracks side by side, while SMR (shingled magnetic recording) partly overlaps them to increase density; changing data on an SMR disk means rewriting neighbouring tracks, so sustained or scattered writes slow down sharply once its internal cache is full. In a single-disk recorder with mostly sequential writes the difference can be small, but in RAID arrays SMR disks can make rebuilds extremely slow and may even be dropped from the array. For multi-disk recorders and RAID, choose CMR disks and check the recording technology in the manufacturer's datasheet before buying.

For recording, rotational speed matters much less than firmware and workload rating. Even a 5400 rpm-class surveillance disk sustains well over 50 MB/s (400 Mbps) of sequential writing across the whole disk, far more than a typical NVR writes: 32 cameras at 4 Mbps are only 128 Mbps, or 16 MB/s. 7200 rpm disks help in large RAID arrays, in faster rebuilds and under heavy simultaneous playback and search, at the cost of more heat, power and noise. In a small NVR, a cooler 5400 rpm-class surveillance disk is a perfectly good choice.

It is not recommended for recordings that may serve as evidence, because the remaining life of a used disk is unknown and 24/7 recording is a demanding workload. If you must use one, check its SMART data first: power-on hours (attribute 9) show its age, and any reallocated (attribute 5) or pending (attribute 197) sectors are a reason to reject it; then run a full (long) self-test before installation. Buy only from a trusted seller, since the SMART counters of resold disks are not always reliable.