⚠️ The Bottom Line

For NAS and RAID: Always choose CMR. SMR drives can cause severe performance problems, failed RAID rebuilds, and data loss risks in multi-drive environments. SMR is only acceptable for light desktop use, external backups, and archival storage where write performance doesn't matter.

Understanding CMR and SMR

CMR (Conventional Magnetic Recording) writes data tracks side-by-side without overlap. Each track can be rewritten independently without affecting neighbors. This is the traditional method used in hard drives for decades.

SMR (Shingled Magnetic Recording) overlaps data tracks like roof shingles to increase capacity. While this allows more data per platter, rewriting any data requires rewriting multiple overlapped tracks — dramatically slowing write operations.

Manufacturers adopted SMR to increase drive capacity without major R&D investment. Unfortunately, they didn't always clearly label SMR drives, leading to the infamous "SMR scandal" when users discovered NAS-rated drives using SMR.

How CMR and SMR Work

Aspect	CMR (Conventional)	SMR (Shingled)
Track Layout	Side-by-side, no overlap	Overlapping like shingles
Write Process	Direct overwrite of single track	Must rewrite entire "zone" of tracks
Read Speed	Fast and consistent	Fast and consistent
Write Speed (Fresh)	Fast and consistent	Fast (uses cache)
Write Speed (Sustained)	Consistent performance	Severe slowdown after cache fills
Random Writes	Good performance	Very poor performance

CMR vs SMR: Complete Comparison

Factor	CMR	SMR	Winner
Sustained Write Speed	Consistent 150-250 MB/s	Drops to 10-50 MB/s after cache	CMR ✓
Random Write Performance	Good	Very Poor	CMR ✓
RAID Rebuild Time	Hours (normal)	Days or fails entirely	CMR ✓
NAS Suitability	Excellent	Not Recommended	CMR ✓
Read Performance	Fast	Fast	Tie
Capacity per Platter	Standard	10-15% higher	SMR ✓
Price	Standard pricing	Sometimes cheaper	SMR ✓
Best For	NAS, RAID, active use	Archives, external backup	Depends on use

Why SMR Causes Problems

SMR's overlapping tracks create a fundamental write penalty:

The Cache Trap

SMR drives have a CMR "media cache" area where writes initially land at full speed. This makes the drive seem normal during light use. But when the cache fills (during sustained writes, RAID rebuilds, or heavy NAS use), performance collapses:

Write speeds drop from 150+ MB/s to 10-50 MB/s
The drive becomes unresponsive for seconds at a time
RAID controllers may time out and mark the drive as failed
RAID rebuilds that should take hours can take days — or never complete

RAID Rebuild Disaster

RAID rebuilds involve massive sustained writes. On SMR drives:

Rebuild times extend from hours to days
During this extended window, another drive failure means data loss
Some rebuilds fail entirely when drives timeout
This defeats the entire purpose of RAID redundancy

This is why NAS and RAID users must verify CMR before purchasing any hard drive.

When CMR vs SMR is Appropriate

✓ Use CMR For:

NAS systems (any size). RAID arrays. Surveillance systems. Active desktop storage. Any write-heavy workload. Professional use. Plex servers.

Required for Reliability

⚠ SMR is Acceptable For:

External backup drives (write once, read occasionally). Cold archive storage. Budget secondary desktop drive for media files. Single-drive use with light writes.

Limited Use Cases

✗ Never Use SMR For:

NAS systems. RAID arrays. ZFS pools. Surveillance DVRs. Any multi-drive setup. Write-heavy workloads. Professional storage.

Risk of Data Loss

When In Doubt: CMR

If you're unsure about your use case, choose CMR. The small price premium is worth avoiding potential disasters. NAS-rated drives are always CMR.

Safe Choice

How to Identify CMR vs SMR Drives

Manufacturers don't always clearly label recording technology. Here's how to check:

Guaranteed CMR Product Lines

Seagate IronWolf: All models CMR
Seagate Exos: All models CMR
WD Red Plus: All models CMR (note: regular "WD Red" without "Plus" is SMR!)
WD Red Pro: All models CMR
WD Ultrastar: All models CMR
Toshiba N300: All models CMR
Toshiba X300: All models CMR

Known SMR Product Lines (Avoid for NAS)

WD Red (non-Plus): 2TB-6TB models use SMR
Seagate Barracuda: Most models use SMR
WD Blue: Many models use SMR
Most external/portable drives: Typically SMR

How to Verify

Check manufacturer's product specifications page
Look for "CMR" or "PMR" (both mean conventional recording)
Search "[model number] CMR or SMR" — enthusiast communities have documented most drives
If specs don't mention recording type, assume SMR and avoid for NAS

CMR NAS Hard Drives — Sorted by $/TB

All NAS-rated drives use CMR technology. Safe for RAID and NAS use.

Product	Capacity	Price	$ / TB	Price Drop	Brand	Interface

Frequently Asked Questions: CMR vs SMR

What does CMR and SMR mean?

CMR (Conventional Magnetic Recording) writes data in non-overlapping tracks that can be modified independently. SMR (Shingled Magnetic Recording) overlaps tracks like roof shingles for higher capacity, but requires rewriting entire zones when modifying data. CMR provides consistent performance while SMR has severe write penalties after its cache fills. For NAS and RAID, CMR is required.

Can I use SMR drives in a NAS?

Technically yes, but strongly not recommended. SMR drives work initially but cause problems under sustained writes — exactly what NAS systems do. RAID rebuilds can take days or fail entirely. Drive timeouts can corrupt arrays. Multiple users have lost data using SMR in NAS/RAID. For NAS, always use NAS-rated CMR drives like Seagate IronWolf or WD Red Plus.

Is WD Red CMR or SMR?

Regular WD Red (non-Plus) uses SMR in 2TB-6TB capacities. This caused significant controversy when discovered. WD Red Plus uses CMR — always choose Red Plus for NAS. WD Red Pro also uses CMR. The naming is confusing by design, unfortunately. For WD NAS drives, look specifically for "Red Plus" or "Red Pro" to ensure CMR technology. See our WD Red Plus guide.

Is Seagate IronWolf CMR?

Yes, all Seagate IronWolf drives use CMR. Seagate has been consistent about using CMR in their NAS line. Both IronWolf (standard) and IronWolf Pro use conventional magnetic recording. This makes IronWolf a safe choice for NAS and RAID use. Seagate Exos enterprise drives are also all CMR. Seagate Barracuda consumer drives, however, typically use SMR.

Why do manufacturers use SMR?

SMR increases capacity by 10-15% without adding platters. This lets manufacturers offer higher capacities at lower costs. For many consumer use cases (external backups, light desktop use), SMR works fine. The problem is when manufacturers used SMR in NAS-rated drives without clear disclosure — leading to failures in use cases where SMR is inappropriate. Transparency has improved, but buyers must still verify.

How can I tell if my drive is CMR or SMR?

Check the manufacturer's product page or datasheet. Look for "CMR" or "PMR" (Perpendicular Magnetic Recording — same as CMR) in specifications. If not listed, search the model number online — enthusiast communities have documented most drives. You can also test sustained write performance: if speeds collapse after a few minutes, it's likely SMR. NAS-rated drives (IronWolf, Red Plus, N300) are always CMR.

Is SMR slower than CMR?

For reads and light writes: No. For sustained writes: Dramatically yes. SMR drives have a CMR cache area that handles initial writes at full speed. Reading is identical to CMR. But when the cache fills during heavy writing, speeds can drop from 150+ MB/s to 10-50 MB/s. This makes SMR unsuitable for write-heavy tasks like RAID rebuilds, large file copies, or NAS operation under load.

What is PMR? Is it the same as CMR?

PMR and CMR refer to the same conventional recording method. PMR (Perpendicular Magnetic Recording) describes how bits are oriented on the platter. CMR (Conventional Magnetic Recording) describes the non-overlapping track layout. Both terms indicate traditional recording without SMR's shingled overlap. If specs say PMR or CMR, the drive is safe for NAS use. Only SMR (Shingled Magnetic Recording) causes the write performance issues.