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StashRaid

Storage Parameters

Usable Capacity

12 TB

Fault Tolerance

4

Array Resilience

Architecture

RAID 5 • Parity

Fault Tolerance

1 Concurrent Failures Allowed

Current Configuration

RAID 5 (Parity)

12 TB Usable

Total RAW: 16 TB

Disk 01DATA
Disk 02DATA
Disk 03DATA
Disk 04PARITY

Total Drives

4 x 4 TB

Efficiency

75% Utilized

Usable: 12 TBWasted to redundancy: 4 TB

Pros & Cons

  • PRO: Good balance of capacity/speed/safety

  • PRO: Cost-effective redundancy

  • CON: Slower writes due to parity calculation

    This setup prioritizes redundancy but has associated performance or capacity costs.

CMR vs SMR: The Most Critical Choice in RAID Drives

If you take away only one piece of advice from this entire site, let it be this: never put SMR drives in a RAID array. Shingled Magnetic Recording technology silently destroys RAID rebuilds, and drive manufacturers have been caught hiding it in NAS-branded products. This guide will teach you exactly what CMR and SMR are, why SMR fails catastrophically in RAID, which specific models to avoid, and which CMR drives to buy instead.

What Are CMR and SMR?

CMR — Conventional Magnetic Recording

CMR writes data to parallel, non-overlapping tracks on the platter. Each track is physically independent — writing to track 42 does not affect track 41 or 43. This means the drive head can write data to any location at full speed, without touching neighboring tracks. CMR has been the standard since hard drives were invented and remains the only safe option for random-write workloads like RAID.

SMR — Shingled Magnetic Recording

SMR overlaps tracks like roof shingles — track 2 partially covers track 1, allowing ~20-25% more data on the same platter. But here's the problem: writing to track 2 forces a rewrite of track 1 underneath it, which forces a rewrite of the next track, and so on. The drive has an internal CMR "cache zone" (~20-30 GB) that masks this penalty for short writes, but once that cache fills, write speeds collapse from 150 MB/s to 2-5 MB/s.

Why SMR Drives Fail Catastrophically in RAID Rebuilds

The Rebuild Death Spiral

When a drive fails in a RAID 5 or 6 array, the controller must rebuild a replacement drive by reading every single sector from the remaining drives and writing the reconstructed data to the new drive. This is a massive, sustained write operation — exactly what SMR cannot handle.

  1. Cache exhaustion: The SMR drive's ~20 GB CMR cache fills within 2-3 minutes of a rebuild.
  2. Write speed collapse: Without the cache, write speeds plummet from 150 MB/s to 2-5 MB/s as the drive struggles with shingled track rewrites.
  3. Controller timeout: The RAID controller (or software RAID like ZFS/mdadm) expects drives to respond within a timeout window. When the SMR drive stalls for 30+ seconds on a write, the controller marks it as dead.
  4. Array destruction: In RAID 5, losing a second drive during rebuild destroys the array. The controller just kicked your only remaining redundancy out of the pool.

A 4TB SMR rebuild that should take 8 hours can stretch to 5+ days — with the array running degraded and unprotected the entire time.

This isn't theoretical. In 2019-2020, Western Digital faced a class-action lawsuit after shipping SMR drives under the WD Red NAS brand without disclosure. Users in the r/DataHoarder and Synology communities documented repeated array failures when WD Red SMR drives were used in RAID 5/6 pools. WD eventually settled and created separate "WD Red Plus" (CMR) and "WD Red" (SMR) product lines.

Drive Models to AVOID — Confirmed SMR

Brand / SeriesModel NumbersWhy It's a Trap
WD Red (non-Plus)WD20EFAX, WD40EFAX, WD60EFAXSold as "NAS drives" but secretly switched to SMR in 2019. WD Red Plus is the CMR version.
Seagate Barracuda ComputeST8000DM004, ST6000DM003, ST4000DM004Cheapest 8TB drives on the market for a reason — 100% SMR. Marketed for desktops but often bought for NAS due to price.
Toshiba P300 DesktopHDWD130, HDWD260, HDWD240Consumer desktop drives with SMR. Often mistaken for NAS drives due to similar pricing.
WD Blue 3.5"WD40EZAZ, WD60EZAZ, WD80EAZZConsumer desktop SMR drives. Fine for single-drive backups, disastrous in RAID.
Seagate Archive HDDST8000AS0002, ST5000AS0011The original SMR consumer drives. Explicitly "Archive" but sometimes found in used NAS bundles.

How to Identify CMR vs SMR Before Buying

1. Check the Manufacturer Datasheet

Every drive manufacturer publishes a product datasheet (PDF). Look for the recording technology section — if it says "CMR" or "Conventional Magnetic Recording," you're safe. If SMR isn't mentioned but the word "archive" or "shingled" appears anywhere, avoid it.

2. Search the Model Number + "CMR SMR"

Websites like NASCompares.com and ServeTheHome maintain verified CMR/SMR lists for every consumer drive model. Search [model number] CMR SMR to find the definitive answer.

3. Buy NAS-Specific Product Lines

NAS-optimized series are consistently CMR: Seagate IronWolf (all), WD Red Plus and Red Pro, Toshiba N300. If a drive is branded for "desktop," "surveillance," or "archive," assume SMR until proven otherwise.

4. Check the Cache Size (Heuristic)

SMR drives often have unusually large DRAM caches (256 MB+) to compensate for shingled write penalties. This isn't definitive — some CMR drives also have large caches — but combined with other checks it's a useful red flag.

Recommended CMR NAS Drives (Amazon Affiliate Links)

All drives listed below are confirmed CMR and specifically designed for 24/7 NAS operation with vibration protection (RV sensors), TLER/ERC error recovery, and firmware optimized for RAID workloads.

Best Pick

Seagate IronWolf Pro 4TB

~$199

7200 RPM, 256 MB cache, 300 TB/year workload rating, 2.5M hours MTBF. Includes 3-year Seagate Rescue Data Recovery. The gold standard for 4-8 bay NAS builds. IronWolf Health Management integrates directly with Synology DSM and QNAP QTS.

View on Amazon →
Budget Pick

Seagate IronWolf 4TB (non-Pro)

~$169

5400 RPM, 256 MB cache, 180 TB/year workload rating, 1M hours MTBF. Quieter and lower power than the Pro model. Excellent choice for home NAS with 1-4 bays where absolute speed isn't critical. Still CMR and fully RAID-compatible.

View on Amazon →
Great Value

WD Red Plus 4TB (WD40EFPX)

~$95

5400 RPM, 256 MB cache, 180 TB/year workload rating. This is the CMR version — make sure you buy "Red Plus" not plain "Red." NASware 3.0 firmware with 3D Active Balance Plus and TLER. The most popular home NAS drive at the best price-per-terabyte.

View on Amazon →
Enterprise-Ready

WD Red Pro 4TB (WD4003FFBX)

~$135

7200 RPM, 256 MB cache, 300 TB/year workload rating, 2.5M hours MTBF. WD's answer to IronWolf Pro. Optimized for 8-24 bay enterprise NAS with vibration protection for dense enclosures. 5-year limited warranty.

View on Amazon →

Pro tip: Always check the exact model number when buying on Amazon — sellers sometimes mix inventory. The model numbers listed above (e.g., WD40EFPX for WD Red Plus 4TB CMR) are the ones you want. Avoid any listing that says "WD Red" without "Plus" or "Pro." When in doubt, check the NASCompares CMR/SMR list before clicking buy.

CMR vs SMR FAQ

What is the difference between CMR and SMR hard drives?

CMR (Conventional Magnetic Recording) writes data to parallel, non-overlapping tracks — each track is independent. SMR (Shingled Magnetic Recording) overlaps tracks like roof shingles to increase density (~20-25% more), but writing to one track forces rewriting adjacent tracks. This is fine for sequential write-once workloads (surveillance DVRs, cold archives) but catastrophic in RAID rebuilds where sustained random writes cause 50-100x slowdowns.

Why do SMR drives fail during RAID rebuilds?

When a RAID array rebuilds, it writes massive amounts of reconstructed data to the replacement drive. An SMR drive's write performance collapses from ~150 MB/s to 2-5 MB/s after its ~20 GB CMR cache fills. The RAID controller interprets this stall as a dead drive and drops it from the array — causing a second failure that can destroy a RAID 5 pool entirely.

Which specific drive models are SMR and should I avoid?

The most dangerous is WD Red (non-Plus) — WD20EFAX, WD40EFAX, WD60EFAX — which were secretly switched to SMR and branded as NAS drives. Also avoid Seagate Barracuda Compute (ST8000DM004, ST6000DM003), Toshiba P300 Desktop, and WD Blue 3.5". Always buy NAS-specific lines: Seagate IronWolf, WD Red Plus/Pro, or Toshiba N300.

How can I tell if a drive is CMR or SMR before buying?

Check the manufacturer's datasheet for the recording technology. Search the model number plus "CMR SMR list" on NASCompares.com or ServeTheHome — they maintain verified databases. Buy NAS-specific product lines (IronWolf, Red Plus/Pro) which are consistently CMR. If a drive is marketed for "desktop," "surveillance," or "archive," assume SMR until you verify otherwise.

Can I use SMR drives in a RAID array at all?

Technically yes, but strongly discouraged. SMR drives may work in RAID 1 mirrors or Unraid configurations where writes are less aggressive. For any parity RAID (RAID 5/6, SHR, RAID-Z), SMR drives cause severe performance degradation and a high risk of array failure during rebuild. The small cost savings (typically $10-20 per drive) is never worth the reliability risk. Always spend the extra few dollars for CMR.