The single most important spec on a hard drive listing is the one retailers barely ever print: whether the drive records with CMR (conventional magnetic recording) or SMR (shingled magnetic recording). Two drives can share a shelf, a price, and a capacity — and behave completely differently the day a RAID rebuild starts.
CMR writes each data track side by side, so any track can be rewritten independently. Write performance is predictable at any queue depth, forever.
SMR overlaps tracks like roof shingles to squeeze in more capacity. Rewriting one track disturbs its neighbours, so the drive must buffer writes in a small CMR-staged cache region and shuffle them into shingled zones during idle time. Under light, bursty desktop use you may never notice. Under sustained writes the staging cache fills — and throughput falls off a cliff, from ~180 MB/s to single-digit MB/s, with multi-second command latencies.
A rebuild (or ZFS resilver) is the worst workload an SMR drive can face: hours of sustained, partly random writes with no idle time to flush the cache. Documented results range from rebuilds taking days instead of hours to drives timing out under load and being kicked from the array entirely — turning one failed disk into a degraded pool with a second "failed" member. That's not a speed annoyance; it's a data-availability risk, arriving at the exact moment your redundancy is already reduced.
EFAX models) without disclosing it. Users found out when
resilvers stalled. The backlash created the CMR-only WD Red Plus line and pushed most vendors
to publish recording-tech tables. The lesson stands: NAS branding is not proof of CMR — check
the model number.| Model family | SMR examples | CMR alternative |
|---|---|---|
| WD Red (plain, 2–6TB) | WD20EFAX–WD60EFAX | WD Red Plus (EFRX/EFZX) |
| Seagate BarraCuda 3.5" (2–8TB) | ST4000DM004, ST8000DM004 | IronWolf (ST4000VN006…) |
| Toshiba P300 (4–6TB) | HDWD240UZSVA | Toshiba N300 |
| Most 2.5" laptop drives ≥1TB | nearly all current models | use an SSD instead |
Enterprise and NAS-Pro lines (Exos, Ultrastar, IronWolf/Pro, WD Red Plus/Pro, Toshiba MG/N300, SkyHawk) are CMR across current capacities.
What SMR is not fine for: ZFS/RAID arrays, surveillance recording, torrents/usenet scratch space, database or VM storage — anything with sustained or random rewrites.
Every drive on the price board carries a CMR or SMR badge — resolved from a curated model database first, then per-family classification rules (a dashed badge means family-inferred rather than exact-model-confirmed). The value score multiplies SMR drives by ×0.45, so a shingled drive has to be more than twice as cheap per TB to rank alongside a CMR equivalent — which is roughly the discount you should demand before putting one anywhere near an array. Tick “CMR only” on the board to hide the trap entirely.