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CMR vs. SMR Hard Drives: Tell Them Apart by Model Number for NAS, RAID and Backup

Two hard drives with the same capacity and series name can differ in recording technology, and CMR versus SMR changes what each drive is suited for. This guide uses the manufacturers' own per-model recording-technology data and workload guidance to choose for NAS, RAID or single-drive backup, then verify the model number before ordering.

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CMR vs. SMR Hard Drives: Tell Them Apart by Model Number for NAS, RAID and Backup overview: 1. CMR and SMR differ in how they write and in the work they suit, 2. Western Digital: tell WD Red (SMR) from WD Red Plus (CMR) by model number, 3. Seagate: check each product family in the official recording-technology list, 4. Choose by use: CMR for RAID and ZFS, SMR conditionally for single-drive backup, 5. How to check a drive you already own, and what to do if it is SMR, 6. Before ordering: line up model number, recording technology, warranty and cost per TB
An overview of this guide’s steps and checks, not a screenshot of the app.

Who this guide is for and what to prepare

  • People choosing drives for a NAS or RAID who want to know whether to avoid SMR
  • People buying an internal or external backup drive who want to confirm the recording technology from the model number

What you need

  • The intended use (NAS RAID, ZFS, PC data storage, or a single external backup drive) and required capacity
  • The full model number of each candidate drive (for example WD40EFPX or ST4000DM004) and the manufacturer's specification PDF
  • The NAS vendor's compatible-drive list if the drive goes into a NAS

1. CMR and SMR differ in how they write and in the work they suit

CMR (Conventional Magnetic Recording) writes tracks without overlap. SMR (Shingled Magnetic Recording) overlaps tracks like roof shingles to gain capacity, and PC and NAS products on the market use the device-managed form (DMSMR), in which the drive itself manages the rewriting internally. Western Digital describes these drives as intended for small-scale workloads based on short periods of access, and notes that they prefer idle time to perform background operations.

The company further explains that sustained random writes during a ZFS resilver (rebuild) leave DMSMR drives without enough idle time, and directs ZFS users to the CMR-based WD Red Plus and WD Red Pro. Seagate likewise states that SMR is best suited to workloads that are sequential or easily sequentialized, such as large media repositories, backup tiers and archives where data is written once and read occasionally. For RAID rebuilds and frequent rewriting, the manufacturers themselves recommend CMR, which is the starting point for the decision.

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1. CMR and SMR differ in how they write and in the work they suit
UseDecision based on manufacturer guidanceBasis
NAS RAID or ZFS (rebuilds occur)Choose CMRWD: DMSMR lacks idle time during ZFS resilvering; use the CMR WD Red Plus or Pro
PC data storage with frequent rewritesCMR is the safer choiceWD: DMSMR targets small-scale workloads with short access periods and needs idle time
Single external backup or archiveSMR is also a candidateSeagate: SMR suits sequential-like backup tiers and archives
  1. Classify your use as a RAID or ZFS NAS, internal PC storage, or a single external backup drive.
  2. If the drive will be used in RAID or ZFS, or rewritten frequently, shortlist only CMR products at this point.
  3. If it is a single backup drive that is written once and mostly read, keep SMR in the running and compare on price and warranty.

2. Western Digital: tell WD Red (SMR) from WD Red Plus (CMR) by model number

In a June 2020 blog post Western Digital stated that the 2TB, 3TB, 4TB and 6TB WD Red drives use DMSMR and that CMR-based NAS drives would be separated as WD Red Plus. Official product briefs checked on September 13, 2026 list WD Red (September 2023 edition) as four models, WD20EFAX, WD30EFAX, WD40EFAX and WD60EFAX, all SMR (device-managed), while WD Red Plus (March 2025 edition) lists ten models from WD20EFPX to WD120EFGX, all CMR.

Both series share the same specification-sheet figures: a 180TB per year workload rate, 1,000,000 hours MTBF, up to 8 bays and a 3-year warranty. Because the numbers match while the recording technology differs, check the model suffix (EFAX versus EFPX, EFZX, EFGX or EFBX) rather than the series name and capacity. WD's July 2020 list also identifies the 3.5-inch WD Blue 2TB (WD20EZAZ) and 6TB (WD60EZAZ), the 2.5-inch WD Blue 1TB and 2TB, and the 2.5-inch WD Black 500GB and 1TB as SMR. That list reflects the model lineup at the time, so verify current products against each product's specification sheet.

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2. Western Digital: tell WD Red (SMR) from WD Red Plus (CMR) by model number
Series (specification edition)Model numbersRecording technologyShared specification figures
WD Red (September 2023)WD20EFAX, WD30EFAX, WD40EFAX, WD60EFAXSMR (device-managed)180TB/year, 1,000,000 hours MTBF, up to 8 bays, 3-year warranty
WD Red Plus (March 2025)WD20EFPX, WD40EFPX, WD60EFPX, WD60EFZX, WD80EFPX, WD80EFZZ, WD101EFBX, WD100EFGX, WD120EFBX, WD120EFGXCMR180TB/year, 1,000,000 hours MTBF, up to 8 bays, 3-year warranty
WD Blue 3.5-inch (July 2020 list)WD20EZAZ, WD60EZAZSMRThat document states all other capacity points use CMR; recheck current models in the spec sheet
  1. Record the full model number from the retailer page and do not choose by series name (WD Red or WD Red Plus) and capacity alone.
  2. Match the model number against the Recording technology row in the manufacturer's specification PDF to settle CMR or SMR.
  3. Where the same capacity has a price gap, compare recording technology, spindle speed, cache and warranty years side by side.

3. Seagate: check each product family in the official recording-technology list

Seagate publishes a page listing CMR or SMR by product family and capacity. As checked on September 13, 2026, the 3.5-inch BarraCuda is SMR at 4TB and 8TB and CMR from 12TB up, the NAS-oriented IronWolf and IronWolf Pro are CMR at every listed capacity, and the surveillance SkyHawk is CMR from 1TB to 8TB. The 2.5-inch BarraCuda (2TB and 4TB) and FireCuda (1TB and 2TB) are SMR. The data-center Exos family is mostly CMR with SMR variants at some of the largest capacities.

The list is by capacity, so for products with more than one model number at the same capacity, confirm the model number against the recording-technology entry in the product manual (PDF) before ordering.

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3. Seagate: check each product family in the official recording-technology list
Seagate product family (3.5-inch)Listed as CMRListed as SMR
BarraCuda12TB, 16TB, 20TB, 24TB4TB, 8TB
IronWolf (NAS)2TB, 4TB, 6TB, 8TB, 10TB, 12TB, 16TBNone listed
IronWolf Pro2, 4, 6, 8, 10, 12, 16, 20 and 24TB, plus Mozaic 28, 30 and 32TBNone listed
SkyHawk (surveillance)1TB, 2TB, 4TB, 6TB, 8TBNone listed
2.5-inch BarraCuda / FireCudaNone listedBarraCuda 2TB and 4TB, FireCuda 1TB and 2TB
  1. Check whether the candidate family and capacity appears in the CMR or the SMR column of Seagate's list.
  2. Search the model number from the retailer page (for example ST4000DM004) in Seagate's product manual and confirm the recording-technology entry matches.
  3. For NAS use, pick from the IronWolf or IronWolf Pro capacities that appear as CMR in Seagate's list.

4. Choose by use: CMR for RAID and ZFS, SMR conditionally for single-drive backup

For a NAS running RAID, for ZFS, or for shared storage with continuous reads and writes by several people, follow the manufacturers' guidance and choose CMR NAS drives (WD Red Plus or Pro, IronWolf or IronWolf Pro). If the NAS vendor publishes a compatible-drive list, check it first. For a single external drive or a monthly full backup, where you write large amounts sequentially and then mostly read, SMR products can also be candidates after comparing price and warranty.

If you choose an SMR drive, expect overwriting on a nearly full drive or rewriting many small files to take longer, and do not power off until the job completes. This guide includes no hands-on speed measurements and does not state specific transfer-rate differences.

Scroll horizontally to see the full table →

4. Choose by use: CMR for RAID and ZFS, SMR conditionally for single-drive backup
How the drive is usedRecording technology to chooseHow to find candidates
NAS RAID or ZFS, always-on shared storageCMRWD Red Plus / Red Pro, Seagate IronWolf / IronWolf Pro, checked against the NAS vendor's list
Internal PC data drive, frequently rewritten working storageCMRConfirm CMR per model even for WD Blue / Black and BarraCuda (BarraCuda 4TB and 8TB are SMR)
Single external drive, periodic full backups, archiveCMR or SMRWith SMR, use it for mostly sequential writes and compare on price, warranty and cost per TB
Surveillance recordingCMR (purpose-built products)Check the recording technology and supported camera count in the lists for products such as Seagate SkyHawk
  1. Pick the row for your use in the table below and confirm in the specification sheet that the candidate model meets that row's condition.
  2. If the NAS vendor has a compatible-drive list, add appearance on that list as a condition alongside the recording technology.

5. How to check a drive you already own, and what to do if it is SMR

Western Digital's support article explains that WD and HGST drives built before 2018 use CMR and that the build date is on the drive label. For later products, match the model number against the specification sheet for the series (WD Red, Blue, Black, Purple, Gold and so on). Seagate drives can be checked the same way with the list and the product manual.

If an SMR drive is already in a RAID, that does not mean it will fail soon, but rebuilds may take longer and the NAS may flag it as not recommended. Until a replacement is scheduled, you can move it to a single-drive backup or archive role.

  1. Read the model number in Windows Disk Management or the NAS management interface, and note the build date on the label.
  2. Match the model number against the manufacturer's specification sheet or list and record CMR or SMR.
  3. If an SMR drive is in a RAID, plan to replace it with CMR at the next swap and keep important data in a separate backup meanwhile.

6. Before ordering: line up model number, recording technology, warranty and cost per TB

Among candidates with the same recording technology, compare cost per terabyte, warranty years, the rated annual workload, spindle speed and cache. This guide includes no retail price survey, so check prices including tax at retailers on the same day. External drives often do not disclose the internal model number, so if recording technology is a requirement, consider buying an internal drive whose spec sheet you can check and fitting it into an enclosure yourself.

Scroll horizontally to see the full table →

6. Before ordering: line up model number, recording technology, warranty and cost per TB
What to checkWhere to lookEasy to miss
Recording technology (CMR / SMR)Recording technology row in the manufacturer's spec sheet, Seagate's listThe same series and capacity can switch technology by model suffix
Annual workload, MTBF, warrantyProduct specification sheetWD Red and WD Red Plus share the figures; only the recording technology differs
NAS compatibilityNAS vendor's compatible-drive listWhen the list was last updated and whether unlisted drives trigger warnings
  1. For each candidate, write one line with model number, recording technology, capacity, spindle speed, warranty years and same-day price including tax, then drop any whose recording technology does not fit the use.
  2. For a NAS, add the vendor's compatible-drive list; when adding to an existing RAID, also write down the current drives' model numbers and check the NAS vendor's guidance on mixing.
  3. When the drive arrives, confirm the model number on the label matches your order before initializing it or adding it to the RAID.

Limitations and requirements

  • The model-to-technology mapping was read from manufacturer documents on September 13, 2026. Specification sheets are revised, so match the model number against the latest edition before ordering.
  • This guide includes no hands-on measurement of SMR versus CMR speed and does not state specific transfer-rate or rebuild-time differences.
  • Compatible-drive lists and warning behavior differ by NAS vendor. Check the NAS vendor's guidance separately from the recording-technology check.

Frequently asked questions

What is the difference between WD Red and WD Red Plus?

Per the official specification sheets, WD Red (WD20 to WD60EFAX) is SMR and WD Red Plus (WD20EFPX to WD120EFGX) is CMR. Figures such as the 180TB annual workload, 1,000,000 hours MTBF, up to 8 bays and 3-year warranty are the same; the recording technology differs.

Is an SMR drive fine for backups?

Seagate states that SMR suits sequential-like workloads such as backup tiers and archives that are written once and read occasionally. Sustained random writes such as RAID or ZFS rebuilds are the workloads for which Western Digital recommends CMR.

Can I tell whether an external drive is CMR or SMR?

Most external products do not disclose the internal drive's model number. If recording technology matters, the reliable approach is to buy an internal drive you can verify in the spec sheet and put it in an external enclosure.

Sources and verification date

Sources checked: . These sources support the specifications, procedures or prices discussed here. Check each source for applicable conditions and current information.

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