CMR vs SMR hard drives
Also known as: SMR vs CMR, SMR vs CMR drives
CMR (conventional magnetic recording) and SMR (shingled magnetic recording) are two methods hard drives use to write data onto spinning platters.
SMR overlaps data tracks like roof shingles to fit more data at lower cost, but rewriting data means rewriting neighbouring tracks too, which can slow performance dramatically during heavy or sustained writes. CMR writes tracks separately and performs more consistently. While SMR drives are fine for archives and occasional backups, CMR drives are generally recommended for NAS devices, RAID arrays, and busy workloads.
CMR vs SMR hard drives, explained at three levels
At home
CMR is like writing on lined paper with a gap between each line, so you can erase and rewrite any line freely. SMR overlaps the lines slightly, like roof shingles, to fit more on the page. The catch is that rewriting one line smudges the one on top of it, so you must rewrite a whole block. That makes SMR drives cheaper but slower for heavy writing.
At work
CMR and SMR are two ways hard drives record data on their platters. CMR keeps tracks separate and performs consistently. SMR overlaps tracks to fit more data at lower cost, but slows sharply during long, heavy writes. Businesses care because SMR drives in a NAS or RAID array can make rebuilds crawl or fail, so CMR is usually the safer choice there.
At home
Hard drives write data in circular tracks on spinning platters. With CMR, each track sits separately with a small gap, so the drive can rewrite any track freely. SMR overlaps tracks like roof shingles to squeeze in more data. The catch is that rewriting one track disturbs the one overlapping it, so the drive must read and rewrite a whole band of tracks to change a little data, which can make sustained writes much slower.
At work
Drive-managed SMR handles shingling inside the drive, using a fast cache area to absorb writes, then reorganizing bands during idle time. When the cache fills under sustained writing, such as a large backup or a RAID rebuild, performance can collapse to a fraction of normal, and some RAID systems interpret the long delays as drive failure. Host-managed SMR, used in data centres, lets software control writes. Manufacturer datasheets list the recording technology.
At home
Western Digital faced criticism in 2020 after selling SMR drives under its WD Red NAS brand without saying so, and some buyers saw RAID rebuilds fail. Manufacturers now label NAS lines more clearly, such as WD Red Plus and Red Pro for CMR, but consumer and external drives often do not say. In my view, checking the exact model number on the maker's website is worth the minute it takes before buying drives for an array.
At work
Use NAS-rated or enterprise CMR drives for RAID and ZFS arrays, standardize drive models within each array, and account for longer rebuild windows on large drives. SMR suits sequential, write-once workloads, such as archives and cold backup targets, where cost per terabyte matters most. Test sustained write performance for backup targets with realistic job sizes, since a drive that benchmarks well briefly may fail backup windows once its cache fills.
Real-world examples
At home
Carlos bought cheap drives for his home NAS and found RAID rebuilds crawling. He learned they were SMR and replaced them with CMR models.
At work
Nadia's Toronto MSP standardized on CMR NAS drives after an SMR drive caused a client's array rebuild to fail during a weekend outage.
How it compares
| feature | CMR | SMR |
|---|---|---|
| Track layout | Separate tracks | Overlapping tracks |
| Capacity per platter | Lower | Higher |
| Sustained write speed | Consistent | Can drop sharply |
| Best for | NAS, RAID, busy workloads | Archives, occasional backups |
CMR is steadier. SMR is denser and cheaper but slower at heavy writes.
For analysts
In hardware specifications for storage, I'd name the recording technology explicitly. Vague 'NAS-compatible' wording led many buyers astray.
History & origin
Hard drive makers adopted SMR in the 2010s to keep increasing capacity as traditional recording approached physical limits. In April 2020, reports revealed that Western Digital, Seagate, and Toshiba had quietly used SMR in some drives sold for NAS and desktop use without clearly disclosing it. Users found some SMR NAS drives failed to rebuild RAID arrays properly. Western Digital faced class-action lawsuits, created a separate 'WD Red Plus' line using CMR, and agreed to a settlement in 2021. Manufacturers began publishing recording technology in specifications. Newer approaches such as HAMR (heat-assisted magnetic recording) arrived in high-capacity drives in the mid-2020s. In my view, the SMR episode was a lesson in why storage buyers should read the fine print.