What is the difference between SMR and CMR?

CMR (Conventional Magnetic Recording) writes tracks side by side. SMR (Shingled Magnetic Recording) overlaps tracks like roof shingles to fit more data per platter. SMR is cheaper but slower for sustained writes, which makes it risky in NAS and RAID.

Can SMR drives work in RAID?

Technically yes, in practice problematic. During a RAID rebuild an SMR drive can stall under sustained load and get kicked from the array as failed. ZFS resilvering is especially sensitive. For active RAID arrays, only CMR drives are recommended.

How do I identify SMR drives?

Manufacturers don't always label clearly. For WD, 'Red Plus' and 'Red Pro' are CMR, plain 'Red' is SMR. Seagate IronWolf 4TB and up is mostly CMR. Check the exact model number on the manufacturer's site, or run smartctl -i which sometimes reports SMR explicitly.

When is SMR acceptable?

For cold-storage backups, external archive drives, and single-drive backup targets where writes are infrequent. Not for active working data, NAS with frequent writes, or RAID arrays.

What was the WD Red SMR scandal?

In 2020 WD shipped SMR drives under the 'Red' (NAS) line without disclosure. Users reported RAID issues and rebuild failures. After public backlash WD launched 'Red Plus' which is explicitly CMR. Lesson: check the model number, not just the product line.

SMR vs CMR Drives: Which Is Actually Safe for Your NAS?

Updated: 2026-04-29

You buy a NAS drive, plug it in, and suddenly your RAID rebuild takes three times longer than expected – or worse: the drive gets kicked from the array as "failed" under load. Common cause: SMR. This guide breaks down the difference to CMR, why SMR can be dangerous in RAID, and how to avoid the trap when buying.

The short version

CMR (Conventional Magnetic Recording) writes tracks parallel side-by-side. Fast, predictable, RAID-friendly. SMR (Shingled Magnetic Recording) overlaps tracks like roof shingles to cram more TB onto each platter. Cheaper per TB, but slow under sustained load and risky for active RAID arrays. Rule of thumb: In a NAS or RAID, use CMR only. SMR is fine as an external backup drive at most.

How SMR works mechanically

A hard drive writes data in concentric tracks on rotating platters. The write head needs more lateral space than the read head. CMR leaves a safety gap between tracks so that writing one track doesn't disturb its neighbors.

SMR exploits this asymmetry: tracks are placed so close that they partially overlap, like roof shingles. Reading still works fine. Writing doesn't: if you want to modify a track that's already written, all overlapping neighbor tracks have to be rewritten too. That makes random writes extremely slow.

Manufacturers compensate with a small CMR cache zone on the drive. New writes land there first, then get reorganized into the SMR area in the background. As long as the drive has idle time, you don't notice. Under sustained load – exactly like a RAID rebuild – the cache fills up and the drive falls back to slow SMR mode.

Why this breaks RAID

Three concrete scenarios where SMR shoots itself in the foot:

1. RAID rebuilds take forever. When a drive fails and gets replaced, the system writes reconstructed data at full speed to the new drive. With CMR, 16 TB takes around 12-20 hours. With SMR it can stretch to 50+ hours because the CMR cache is permanently full and the drive constantly waits for the background reorganization process.

2. The RAID controller drops the drive. If latency gets too high, the controller marks the drive as "failed". Suddenly you have two "failed" drives in a RAID 5 – the array is dead, even though the drives are physically fine. What happens when RAID fails covers this in detail.

3. ZFS resilver fails. ZFS and RAIDZ are especially sensitive because resilvering generates constant write load. Multiple users on TrueNAS forums have documented SMR drives never finishing resilver, or throwing checksum errors throughout.

The 2020 WD Red scandal

In 2020 it became public that Western Digital was quietly shipping SMR drives in the "WD Red" NAS line – without labeling them. Users reported widespread rebuild problems, ZFS resilver failures, and in worst cases data loss. After lawsuits and public pressure, WD introduced "Red Plus" and "Red Pro" lines explicitly marked as CMR. Plain "Red" (no suffix) is still SMR.

Seagate fended off similar accusations on smaller IronWolf models – some 2 TB and 4 TB batches were DM-SMR. Larger IronWolf and IronWolf Pro were and remain CMR.

Lesson: don't trust the product line, verify the specific model number on the manufacturer's site.

How to spot SMR drives

Check the model on the manufacturer's site. WD maintains an official list, and Seagate too. Toshiba consistently labels their N300 line as CMR.

Check smartctl output. On Linux: sudo smartctl -i /dev/sdX. Some newer SMR drives report themselves as "Host-Managed SMR" or "Drive-Managed SMR" – clear giveaway. For most consumer SMR drives (DM-SMR) it doesn't show up in the output.

Random write benchmark. Most reliable test: write 50-100 GB in random order to the drive. CMR holds steady at 150-250 MB/s. SMR drops to 20-50 MB/s after a few GB. Tools: fio, dd with random data, or for Synology directly via test mode.

Manufacturer status 2026

Western Digital:

WD Red Plus: CMR, NAS-rated, 1-14 TB
WD Red Pro: CMR, NAS Pro line, higher RPM, 4-22 TB
WD Red (no suffix): SMR – avoid for RAID
WD Blue 2-6 TB: often SMR – not for NAS
WD Gold, Ultrastar: CMR, enterprise

Seagate:

IronWolf 4 TB+: CMR (verify smaller capacities)
IronWolf Pro: CMR, NAS Pro line
Exos: CMR, enterprise
Barracuda Compute 2-8 TB: many SMR variants – careful
Skyhawk (surveillance): mostly CMR

Toshiba:

N300: CMR, NAS line, 4-22 TB
MG series: CMR, enterprise
P300: mixed – check the model number

When SMR makes sense

SMR isn't bad in general. It's just misused in NAS. Sensible use cases:

External backup drive. You back up your NAS once a week to an external USB drive. Write load isn't continuous, the drive sits in a drawer for days in between. SMR cache has time to clear.

Cold archive. Photo or video archive that you write once and only read from. SMR read performance is normal.

Single-drive backup target. One drive for periodic incremental backups – no RAID, no parallelism. SMR is often 30-40% cheaper per TB than CMR here.

In all of these you save real money. Just not in active RAID.

Recommendation matrix

For NAS / active RAID: CMR only. WD Red Plus, Red Pro, Seagate IronWolf (Pro), Toshiba N300.

For external backup: SMR is OK if the price is right. WD Elements or Seagate Expansion are usually cheap.

For Plex / media server with mostly read load: Prefer CMR, but SMR works passably after the initial fill. More on Plex storage planning.

For ZFS: Never SMR. Period. ZFS communities are unusually unanimous on this.

To compare actual capacities across different RAID setups, head to the RAID Calculator.

FAQ

Do SMR drives even work in a NAS?
For read-only use, yes. With normal write volume initially fine. During rebuild or under sustained load: high risk that the RAID system tags them as failed.

Can I mix SMR and CMR?
Technically yes, practically pointless. The SMR drive becomes the bottleneck and can cause rebuild issues. If you must mix: mix different sizes, not technologies.

Are all modern WD Red drives CMR?
No. Only "Red Plus" and "Red Pro". Plain "Red" (no suffix) is still SMR today. WD marketing is confusing here, always verify the specific model number.

How big is the performance difference really?
Sequential reads: nearly zero. Sequential writes under sustained load: 3-5× slower. Random writes: 5-20× slower.

Why do manufacturers sell SMR as NAS drives at all?
SMR is 20-30% cheaper to manufacture. As long as buyers don't notice, the margin is higher. The 2020 WD Red scandal made the market a bit more transparent – but only a bit.

Best NAS Drives 2026 – Concrete model recommendations with current pricing.

Finding cheap NAS drives – How to save without falling into the SMR trap.

RAID Complete Guide – Which RAID level fits which setup.

ZFS vs ext4 vs Btrfs – Why ZFS is especially SMR-sensitive.

RAID Calculator