CMR vs SMR: what's the difference?
Updated June 2026
If you're shucking drives for a NAS, this is the one spec that matters. CMR drives are safe for arrays; SMR drives can wreck a rebuild. Here's the plain-English difference and how to make sure you buy the right one.
What the acronyms mean
CMR — conventional magnetic recording. Data tracks are written side by side with gaps between them. Each track can be rewritten independently, so writes are predictable and fast.
SMR — shingled magnetic recording. Tracks are partially overlapped, like roof shingles, to squeeze more capacity onto the same platters. The catch: because tracks overlap, rewriting one can force the drive to read and rewrite the tracks next to it. Under heavy random writes, that bookkeeping piles up and the drive slows dramatically.
Why SMR is bad for a NAS or RAID array
The danger is the rebuild. When a drive in a RAID/ZFS/unRAID array fails and you replace it, the array reconstructs the data with a long, sustained write to the new disk. An SMR drive can stall badly during that process — slowing to a crawl or dropping out — and a failed rebuild can take the whole array with it. That's why the home-server community is adamant: CMR only for arrays.
When SMR is actually fine
SMR isn't evil — it's just specialized. It's perfectly good for cold archival storage and large sequential writes on a single disk: back up a big folder, fill the drive once, read from it often. The problems only show up under sustained random writes and array rebuilds.
How to tell if a shuckable drive is CMR
- Capacity rule of thumb: desktop externals at 12 TB and above ship CMR mechanisms. The shuckable sweet spot is CMR by default.
- Watch the small sizes: some 2 TB and 6 TB units have used SMR — avoid those for an array.
- Verify the exact model: manufacturers publish CMR/SMR per model, and the community maintains lists. For shucking, the capacity rule is usually enough.
Which shuckable drives are CMR?
The drives this site tracks — WD Elements, WD My Book and Seagate Expansion at 12 TB+ — are CMR, which is exactly why they're the ones worth shucking for a NAS. Compare them by live $/TB on the tracker.
CMR vs SMR FAQ
What is the difference between CMR and SMR drives?
CMR (conventional magnetic recording) writes data tracks side by side. SMR (shingled magnetic recording) overlaps tracks like roof shingles to pack in more data, but overlapping means writing one track can force rewrites of neighbors — which makes sustained random writes slow.
Is SMR bad for NAS and RAID?
For an array, yes — avoid it. SMR drives can stall during the sustained random writes of a RAID/ZFS parity rebuild, and a stalled rebuild risks the whole array. Use CMR in any RAID, ZFS or unRAID setup.
When is SMR acceptable?
SMR is fine for cold archival storage and large sequential writes on a single disk — write-once, read-many workloads. It is not suited to busy random-write use or array rebuilds.
How do I know if a shuckable drive is CMR?
As a rule of thumb, desktop external drives at 12 TB and above ship CMR mechanisms; some smaller capacities (notably 2 TB and 6 TB) have used SMR. For certainty, check the manufacturer datasheet or a community CMR/SMR list for the exact model.