CMR vs SMR Drives for NAS: Why It Matters More Than Brand
Compatibility data on this site last refreshed 2026-10-29 · reading time ~3 min
CMR (conventional magnetic recording) and SMR (shingled magnetic recording) describe how a hard drive writes data. For desktop use the difference barely shows. In a NAS — where drives run 24/7 in RAID — it decides whether your array survives its first rebuild.
The problem with SMR in a NAS
SMR drives overlap write tracks like roof shingles to squeeze more capacity onto platters. Random writes and long sustained writes force the drive to re-shingle data, causing write speeds to collapse from ~180MB/s to under 10MB/s for minutes at a time. During a RAID rebuild — exactly when a second drive failing kills the array — that stall can stretch a 10-hour rebuild into days.
Which drive lines are CMR
- Seagate IronWolf and IronWolf Pro: CMR across the line
- WD Red Plus, Red Pro and Gold: CMR
- Toshiba N300 and MG series: CMR
- WD Red (non-Plus): mixed — several part numbers (e.g. the 2-6TB EFAX models) are SMR; check the exact part number before buying
- Desktop lines (WD Blue, Barracuda, Toshiba P300): mostly SMR at 2TB+ — avoid in multi-bay NAS
How to check a specific drive
Search the exact part number, not the marketing name. Synology's official compatibility list doesn't label CMR/SMR, so verify on the vendor's datasheet — or open the drive's page here and check its series: every NAS-tuned series listed on this site (IronWolf, Red Plus, Red Pro, N300, MG, Exos, Gold) is CMR.
Quick answers
Is CMR really necessary for a 2-bay NAS?
It is strongly recommended even in 2-bay units. A 2-bay RAID 1 has zero redundancy during a rebuild — the exact scenario where SMR stalls hurt most.
Are all WD Red drives SMR?
No. WD Red Plus, Red Pro and Gold are CMR. Only some non-Plus WD Red part numbers (mainly 2-6TB EFAX variants) use SMR.
Does Synology block SMR drives?
No. Several SMR part numbers appear on Synology's official compatibility list, which is why checking CMR status yourself matters.
Check your exact model
Compatibility is per machine and per part number. Pick your model for its full official drive list: