SMR drives hiding in NAS lines
Manufacturers have quietly shipped SMR drives into NAS and desktop lines. Why it wrecks RAID rebuilds, which current models are SMR, and how to avoid them.
The most important spec when you buy a drive for a NAS is the one manufacturers are quietest about: whether it is CMR or SMR. Get it wrong and the drive can stall and drop out of a RAID array mid-rebuild, taking your redundancy with it. For a few years, the makers got it wrong on purpose.
The WD Red episode
In 2020, people started noticing their WD Red drives - WD's NAS line - choking during RAID rebuilds. WD had quietly switched several Red models (WD20EFAX, WD30EFAX, WD40EFAX, WD60EFAX) to SMR, with nothing on the box or the spec sheet to say so. It ended in a class action and a renamed lineup: the CMR drives became Red Plus, and plain "Red" was left as the SMR budget option. The lasting lesson wasn't the lawsuit, it was this: the word "NAS" in a product name guarantees nothing, and the label won't tell you the recording technology.
Seagate and Toshiba did the same thing more quietly in their desktop lines. The box still won't tell you.
Why SMR is dangerous in an array
SMR (shingled recording) overlaps tracks like roof tiles to pack in more capacity. The catch is writes: changing any data means rewriting a whole zone. Under the sustained random writes a RAID rebuild or resync throws at a disk, an SMR drive slows to a crawl, the controller times it out, and it gets dropped from the array. If a second drive fails before the rebuild finishes, the array is gone.
So this isn't really about failure rate - an SMR drive can be perfectly reliable sitting in a single-drive desktop. It's that the write-stall is specifically lethal in parity RAID. CMR writes each track independently and keeps up, which is the whole reason it's the RAID-safe default.
Which current drives are SMR
These are the SMR models still on shelves. None of them belong in a multi-bay NAS:
- WD - Blue WD20EZAZ / WD40EZAZ / WD60EZAZ; the older Red WD20EFAX / WD30EFAX / WD40EFAX / WD60EFAX.
- Seagate - BarraCuda ST2000DM008 / ST3000DM007 / ST4000DM004 / ST6000DM003 / ST8000DM004.
- Toshiba - P300 HDWD240 / HDWD260; DT02 DT02ABA400 / DT02ABA600.
All of them are budget desktop lines (or, in the EFAX case, a NAS line that shouldn't have been SMR). Fine for a single backup disk or an archive box. Not for a RAID pool.
How to avoid the trap
- Buy the lines that are explicitly CMR: WD Red Plus or Red Pro (not plain Red), Seagate IronWolf or IronWolf Pro, Toshiba N300, or enterprise drives (Exos, Ultrastar, Toshiba MG).
- Check the exact model number, not the family name - that's where the SMR ones hide.
- Look it up before you buy, not after the array is built.
Every drive on this site is tagged CMR or SMR per model, so you can filter SMR out entirely, scan the full SMR blacklist, or read the CMR vs SMR explainer.