In short: CMR (Conventional Magnetic Recording) writes tracks side by side without overlap and keeps a steady speed under any load. SMR (Shingled Magnetic Recording) overlaps tracks like roof shingles, so it fits more data per platter and costs less, but slows down sharply during long rewrites. NAS, RAID and CCTV need CMR; for a home media library or archive SMR is fine.
How CMR works
With conventional (also called perpendicular, PMR) recording, every track on the platter has its own width and does not touch its neighbours. To change data in one sector, the drive simply rewrites that sector. Write speed therefore barely depends on what the drive did before: small files, large files, random or sequential writes - the behaviour is predictable.
How SMR works
The write head is physically wider than the read head. SMR takes advantage of this: each new track partly overlaps the previous one, leaving only a narrow strip exposed, which is enough for reading. More tracks fit on the same platter, so the manufacturer gets more capacity without extra platters or heads.
The cost is rewriting. You cannot change data in the middle of the "shingles" without disturbing neighbouring tracks, so the drive rewrites a whole group of tracks (a zone). To hide this from the user, SMR drives have a CMR buffer area and a large cache: new data goes there first, and the drive moves it into zones when idle. While the buffer has room, SMR is fast. When writing continues for hours without pauses, the buffer fills up and speed can drop several times.
When SMR is a good choice
- A second drive in a home PC for games, films and photos.
- An archive you fill gradually and mostly read.
- An external drive for weekly backups.
In these scenarios the drive gets idle time to clear its buffer, and you will not notice any difference from CMR. In return you get more capacity for the same money.
When you need CMR only
- NAS and RAID. During an array rebuild the drive writes non-stop for many hours. An SMR drive can slow down so much that the controller considers it unresponsive and drops it from the array. That is why NAS builds use dedicated NAS series with CMR.
- CCTV. A recorder writes 24/7 without pauses. Surveillance drives (WD Purple, Seagate SkyHawk) use CMR.
- Constant rewrites: torrent clients, daily incremental backups, virtual machines.
Which series are CMR and which are SMR
Manufacturers rarely put the recording type in the product name, so go by the part number. The general picture:
| Series | Recording | Note |
|---|---|---|
| WD Red Plus, WD Red Pro | CMR | NAS series |
| WD Red (not Plus), 2-6 TB | SMR | Do not confuse with Red Plus |
| WD Purple | CMR | CCTV |
| WD Blue 3.5" (EZEX, EZAX) | CMR | EZAZ part numbers are SMR |
| WD Blue 2.5" (SPZX) | SMR | Laptop drives |
| Seagate IronWolf, IronWolf Pro | CMR | NAS series |
| Seagate SkyHawk | CMR | CCTV |
| Seagate BarraCuda 2-8 TB | SMR | Desktop series |
| Toshiba P300 2 TB (HDWD120) | CMR | 4 and 6 TB are SMR |
| WD Gold, Ultrastar, Seagate Exos | CMR | Enterprise |
Product lines change, so for a specific drive check the specifications on the product page: we list the recording type for every item in the catalog.
How to choose in 30 seconds
- The drive will run 24/7 or in a multi-drive array - CMR only.
- The drive is rewritten often and for long periods - CMR.
- The drive mostly stores and serves data - SMR will do and costs less.
