TL;DR: CMR drives (conventional recording with no track overlap) are the safe choice for RAID, NAS and databases. SMR drives (overlapping tracks = +20–25 % capacity) are suitable only for WORM archiving; in RAID/NAS they cause slow rebuilds, timeouts and ZFS resilvers that take weeks. WD Red drives (3–6 TB) without the „Plus„ / „Pro" suffix are SMR — in 2020 this triggered a scandal that finally forced the manufacturer to label the technology openly. A list of specific models is below.

A few months ago, a 4-bay NAS arrived in Brno from a partner service centre — the array degraded, one drive dropped, the rebuild stalled on its own at 71 % after six days. In the meantime the client had bought a replacement drive, „the same model that was in there", slotted it into the bay and wondered why the NAS could not cope. In the workshop we found out within an hour that the problem was not the hardware — the array was trying to resilver onto a WD Red SMR drive, and every write past a few GB dropped throughput below 10 MB/s. With a 4 TB resilver that means a week and then some.

This article is about why it works this way and how to spot specific models before you slot them into a NAS or a ZFS pool.

Shelves with the hard drive archive in the ITHOPE Brno laboratory — dozens of 3.5-inch HDD models lined up in workshop slots
Part of the drive archive in our Brno laboratory. CMR and SMR, NAS and desktop, IronWolf next to Red, Exos next to Barracuda. After five years in the workshop you can recognise one by its chassis design — but you cannot read the recording technology off the metal. Only the datasheet will tell you that.

The difference between CMR (Conventional Magnetic Recording) and SMR (Shingled Magnetic Recording) is invisible to the average user, but for performance in RAID arrays, databases and NAS devices it is fundamental. This article summarises the current state (2026) and provides a list of specific models from Seagate, Western Digital, Toshiba and Samsung.

What CMR is

The traditional way of writing: the magnetic tracks on the platter are stored side by side with no overlap. Writing to any track does not change its neighbours. The advantage is consistent performance when writing — whether you write 1 GB or 10 TB, the speed does not change. Sustained write throughput holds its 150–250 MB/s depending on the model, with no drops.

What SMR is

„Shingled„ means „like roof shingles". The tracks partially overlap, which increases capacity by 20–25 % on the same physical disk. The price: writing to one track requires rewriting the whole „zone" of neighbouring tracks. The drive therefore internally uses a large „media cache" (a CMR area, typically 20–30 GB) and later moves the data into the shingled area in the background.

The consequence in numbers:

  • Sequential write of the first ~25 GB: 150–180 MB/s (running into the CMR cache).
  • Once the cache is exhausted: a drop below 10 MB/s, sometimes even 3 MB/s, until the cache empties.
  • Random write (4 K random): on SMR often five to ten times slower than on the CMR equivalent.
  • During a RAID resilver / ZFS scrub: the throughput drop holds for tens of hours and the drive often expires on a timeout.
An open 3.5-inch hard drive on a green cutting mat, lit by green studio light — visible spindle, platter and head parking ramp
CMR and SMR share the same physical construction — platters, head-stack, spindle. The difference is in how the tracks are arranged under the head. You cannot tell from an open drive; the firmware keeps that inside. (Studio shot for illustration — sharp work on the platters is always done in a laminar flow box.)

Where SMR is a problem

  • RAID rebuild — the array rebuilds for tens of hours up to weeks, and the drive often expires on a timeout (Synology reports „abnormal", mdraid drops the drive from the array). Then you rebuild again from scratch and, above all, without redundancy in the meantime — one more failure = the data is gone.
  • ZFS resilver — SMR drives in a ZFS pool are an anti-pattern (OpenZFS issue #10214). A resilver of a small pool (4× 4 TB) can take 4–7 days instead of 12 hours with CMR. The same problem occurs during a scrub.
  • Databases — a transactional write saturates the cache within minutes, latency rises from 5 ms to 500 ms. PostgreSQL, MySQL, MariaDB — they all suffer the same way.
  • NAS Synology / QNAP / TrueNAS — an SHR / RAID-5 / RAID-Z rebuild on SMR drives is a nightmare. In 2020 Synology officially issued a warning that it would not guarantee WD Red SMR drives for DSM SHR.
  • Surveillance / video recorders — continuous 24/7 writing fills the cache and an SMR drive cannot keep up with the stream. For an NVR use CMR exclusively (Seagate Skyhawk Surveillance CMR, WD Purple, Toshiba S300).

SMR is OK for archival storage, single-drive external backups, WORM (Write Once Read Many) scenarios. Not for production performance scenarios, not for RAID, not for a NAS with a rebuild in live operation.

Screenshot of a RAID forensic tool with the configuration of a virtual RAID 0 (stripe size 16 KB) and a hex view of the boot sector of a reconstructed image
When an SMR array drops and a classic rebuild fails, we read each drive separately outside the NAS, create a RAW image from it and only then assemble the array in software. The picture shows the configuration of a virtual RAID 0 (stripe 16 KB) and a hex check of the boot sector — the moment when we know the data is out.

Identification — how to tell CMR from SMR

Manufacturers historically kept it secret. After the 2020 scandal (WD Red, when Western Digital quietly switched the Red 2–6 TB to SMR and did not tell customers) transparency is now mandatory. You can verify it via:

  • The manufacturer’s datasheet — look for the words CMR, SMR, PMR, shingled, host-aware. WD has a public table (WDC SMR/CMR list); Seagate publishes a datasheet per model.
  • The specification on the retailer’s website (Alza, CZC, Mironet, Amazon) — larger retailers list it in the specification section these days.
  • The smartctl tool — some newer SMR drives report the fields Form Factor: 3.5 inches and Recording technology: SMR in smartctl -a /dev/sdX (if the drive supports host-aware SMR, it reports this consistently). Older drive-managed SMR does not list it — then the only option left is the datasheet.
  • The model designation — with WD Red the distinction is in the suffix: WD60EFAX = SMR, WD60EFRX = CMR (older), WD60EFPX/WD60EFZX = CMR Plus (new). The tables below show all the cases.

Hard drive (HDD) overview — list of models with CMR and SMR technology

Are you planning to buy a new hard drive and not sure whether to go for CMR or SMR technology? We have prepared a detailed list of hundreds of models from leading manufacturers — Western Digital, Seagate and Toshiba — to make your decision easier. The tables are sorted by capacity (from largest to smallest) and secondarily by brand. That way you can easily compare drives in the size you need.

For each entry you will find: the brand (WD, Seagate, Toshiba, HGST), the series (IronWolf, Red Plus, N300, Exos, Ultrastar and others), the exact model designation, the capacity and, above all, whether it is CMR (Conventional Magnetic Recording) or SMR (Shingled Magnetic Recording).

20 TB and 18 TB drives — enterprise class

BrandSeriesModelCapacityTechnology
SeagateExosST20000NM007D20 TBCMR
WDElementsWDBWLG0200HBK20 TBCMR
SeagateExosST18000NM000J18 TBCMR
SeagateExosST18000NM000D18 TBCMR
SeagateExosST18000NM003D18 TBCMR
WDDC HC5500F3845918 TBCMR
WDMyBookWD180EDGZ18 TBCMR
WDElementsWDBWLG0180HBK18 TBCMR
WDRed ProWD181KFGX18 TBCMR
WDWhiteWD180EDFZ18 TBCMR

In this class (18 TB and above), SMR is practically not used today — enterprise drives go the CMR way exclusively because of the requirements for consistent performance in RAID and NAS.

16 TB and 14 TB drives

BrandSeriesModelCapacityTechnology
SeagateExosST16000NM001G16 TBCMR
SeagateIronWolf ProST16000NE00016 TBCMR
ToshibaEnterpriseMG08ACA16TE16 TBCMR
ToshibaMG09MG09ACA18TE16 TBCMR
WDElementsWDBWLG0160HBK16 TBCMR
WDWhiteWD160EDGZ16 TBCMR
WDUltrastarWUH721816ALE6L416 TBCMR
SeagateIronWolfST14000VN000814 TBCMR
ToshibaEnterpriseMG07ACA14TE14 TBCMR
WDHC530WUH721414ALE6L414 TBCMR
WDMyBookWDBBGB0140HBK14 TBCMR
WDRed PlusWD140EFGX14 TBCMR
WDRed ProWD141KFGX14 TBCMR
WDWhiteWD140EDGZ14 TBCMR
WDWhiteWD140EDFZ14 TBCMR

12 TB and 10 TB drives

BrandSeriesModelCapacityTechnology
SeagateIronWolfST12000VN000812 TBCMR
SeagateExosST12000NM00012 TBCMR
SynologyHAT5300HAT5300-12T12 TBCMR
ToshibaEnterpriseMG07ACA12TE12 TBCMR
WDElementsWD120EDBZ12 TBCMR
WDElementsWDBBGB0120HBK-EESN12 TBCMR
WDMyBookWD120EDAZ12 TBCMR
WDMyBookWD120EDGZ12 TBCMR
WDRed PlusWD120EFBX12 TBCMR
WDWhiteWD120EMFZ12 TBCMR
SeagateIronWolfST10000VN000810 TBCMR
SeagateBarracuda ProST10000DMZ00410 TBCMR
SeagateIronWolfST10000VN000410 TBCMR
SeagateIronWolf ProST10000NE000810 TBCMR
SeagateIronWolf ProST10000NE00010 TBCMR
WDEasyStoreWD100EMAZ10 TBCMR
WDElementsWD101EDBZ10 TBCMR
WDElementsWDBWLG0100HBK10 TBCMR
WDGoldWD102KRYZ10 TBCMR
WDRed PlusWD101EFBX10 TBCMR
WDRed PlusWD101EFAX10 TBCMR
WDUltrastar HC330WUS721010ALE6L410 TBCMR
WDWhiteWD100EZAZ10 TBCMR

8 TB drives

BrandSeriesModelCapacityTechnology
SeagateIronWolfST8000VN0048 TBCMR
SeagateBarracuda ProST8000DM00048 TBCMR
SeagateIronWolfST8000VN00228 TBCMR
SeagateExosST8000NM017B8 TBCMR
SeagateSkyhawkST8000VX0028 TBSMR
SeagateDesktopSTGY80004008 TBSMR
ToshibaN300HDWG480UZSVA8 TBCMR
WDBlueWD80EAZZ8 TBCMR
WDWhiteWD80EDBZ8 TBCMR
WDMyBookWD80EDAZ8 TBCMR
WDRed PlusWD80EFZZ8 TBCMR
WDWhiteWD80EMZZ8 TBCMR
WDBlueWD80EZAZ8 TBCMR
WDRed ProWD8003FFBX8 TBCMR
WDUltrastarWD80EMAZ8 TBCMR
WDElementsWDBWLG0080HBK-EESN8 TBCMR
WDGoldWD8004FRYZ8 TBCMR
WDRed PlusWD80EFAX8 TBCMR
WDBlackWD8002FZWX8 TBCMR
WDMyBookWDBBGB0080HBK8 TBCMR
WDMyBookWDBBGB0080HBK-EESN8 TBCMR
WDRed PlusWD80EFZX8 TBCMR

6 TB drives

BrandSeriesModelCapacityTechnology
SeagateIronWolfST6000VN0016 TBCMR
SeagateExos 7E8ST6000NM01156 TBCMR
SeagateIronWolfST6000VN00336 TBCMR
SeagateSkyhawkST6000VX0016 TBCMR
SeagateConstellationST6000NM00446 TBCMR
SeagateSkyhawkST6000VX0106 TBSMR
ToshibaN300HDWG460UZSVA6 TBCMR
ToshibaDesktopHDWD260EZSTA6 TBSMR
ToshibaMG08MG08ADA600E6 TBCMR
ToshibaDesktopHDWD260UZSVA6 TBSMR
WDRed PlusWD60EFPX6 TBCMR
WDRed PlusWD60EFZX6 TBCMR
WDBlackWD6003FZBX6 TBCMR
WDPurpleWD63PURZ6 TBCMR
WDUltrastarHUS726T6TALE6L46 TBCMR
WDPurpleWD60PURZ6 TBCMR
WDGoldWD6002FRYZ6 TBCMR
WDGoldWD6003FRYZ6 TBCMR
WDRedWD60EFRX6 TBCMR

5 TB and 4 TB drives

BrandSeriesModelCapacityTechnology
SeagateBarracudaST5000DM0035 TBSMR
WDElementsWDBU6Y0050BBK5 TBSMR
WDRedWD50EFRX5 TBCMR
WDMy PassportWDBPKJ0050BBK5 TBSMR
WDBlueWD50EZRZ5 TBCMR
SeagateIronWolfST4000VN0084 TBCMR
SeagateIronWolfST4000VN0064 TBCMR
SeagateDesktopST4000DM0004 TBCMR
SeagateSkyhawkST4000VX0074 TBCMR
SeagateSkyhawkST4000VX0164 TBCMR
SeagateSkyhawkST4000VX0134 TBSMR
SeagateIronWolf ProST4000NE0014 TBCMR
SeagateConstellationST4000NM00334 TBCMR
SeagateConstellationST4000NM00234 TBCMR
SeagateSkyhawkST4000VX0004 TBCMR
SeagateSkyhawkST4000VX0054 TBSMR
SeagateIronWolfST4000VNZ064 TBCMR
ToshibaCanvioHDTB440EK3CA4 TBSMR
ToshibaX300HDWR440XZSTA4 TBCMR
ToshibaCanvioHDTX1404 TBSMR
ToshibaEnterpriseMG08ADA400N4 TBCMR
ToshibaX300HDWE140UZSVA4 TBCMR
ToshibaMQ04MQ04ABB4004 TBCMR
ToshibaN300HDWG440EZSTA4 TBCMR
WDRed PlusWD40EFPX4 TBCMR
WDRedWD40EFRX4 TBCMR
WDPurpleWD42PURZ4 TBCMR
WDPurpleWD40PURX4 TBCMR
WDPurpleWD42EJRX4 TBCMR
WDRed PlusWD40EFZX4 TBCMR
WDPurpleWD40PURZ4 TBCMR
WDBlueWD40EZRZ4 TBCMR
WDREWD4000FYYZ4 TBCMR
WDGoldWD4003FRYZ4 TBCMR
WDBlack (PMR)WD4003FZEX4 TBCMR
WDBlueWD40NMZW4 TBSMR
WDRedWD40EFAX4 TBSMR
WDMyBookWDBBGB0040HBK4 TBSMR
WDRed PlusWDBAVV0040HNC4 TBCMR
WDElementsWDBU6Y0040BBK4 TBSMR
WDBlueWD40NPZZ4 TBSMR
WDRed ProWD4003FFBX4 TBCMR
WDElementsWD40NDZW4 TBSMR
HGSTUltrastarHUS724040ALA6404 TBCMR

3 TB and 2 TB drives

BrandSeriesModelCapacityTechnology
SeagateIronWolfST3000VN0073 TBCMR
SeagateDesktopST3000DM0013 TBCMR
SeagateDesktopST3000DM0003 TBCMR
ToshibaP300HDWD130EZSTA3 TBCMR
ToshibaDT01DT01ACA3003 TBCMR
ToshibaP300HDWD130UZSVA3 TBCMR
WDRedWD30EFRX3 TBCMR
WDGreenWD30EZRX3 TBCMR
WDBlackWD3003FZEX3 TBCMR
WDPurpleWD30EJRX3 TBCMR
WDRed PlusWD30EFPX3 TBCMR
SeagateBarracudaST2000DM0062 TBSMR
SeagateBarracudaST2000DM0082 TBSMR
SeagateIronWolfST2000VN0042 TBCMR
SeagatePortableSTKM20004002 TBSMR
SeagateMobileST2000LM0072 TBSMR
ToshibaP300HDWD120UZSVA2 TBCMR
ToshibaDT01DT01ACA2002 TBCMR
WDRed PlusWD20EFZX2 TBCMR
WDBlueWD20EZRZ2 TBCMR
WDRedWD20EFRX2 TBCMR
WDGreenWD20EZRX2 TBCMR
WDBlackWDBA2W0020BBK2 TBCMR
WDPurpleWD20PURZ2 TBCMR
WDGreenWD20EARX2 TBCMR
WDBlackWD2003FZEX2 TBCMR
WDPurpleWD20EJRX2 TBCMR
WDPurpleWD22EJRX2 TBCMR
WDRed ProWD2001FFSX2 TBCMR
WDPurpleWD22PURZ2 TBCMR
WDElementsWDBU6Y0020BBK2 TBSMR
WDBlackWD2002FAEX2 TBCMR
WDBlueWD20EZAZ2 TBSMR

1 TB and smaller drives

BrandSeriesModelCapacityTechnology
SeagateIronWolfST1000VN0021 TBCMR
SeagateBarracudaST1000LM0491 TBSMR
SeagateBarracudaST31000528AS1 TBCMR
SeagateBarracudaST1000DMZ101 TBCMR
SeagateMomentusST1000LM0241 TBSMR
SeagateBarracudaST31000524AS1 TBCMR
ToshibaDT01DT01ACA1001 TBCMR
ToshibaP300HDWD110EZSTA1 TBCMR
ToshibaP300HDWD110UZSVA1 TBCMR
ToshibaLaptop L200MQ041 TBSMR
WDBlueWD10JPVX1 TBCMR
WDRed PlusWD10JFCX1 TBCMR
WDPurpleWD10PURZ1 TBCMR
WDBlueWD10SPZX1 TBSMR
WDGreenWD10EZRX1 TBCMR
WDGreenWD10EURX1 TBCMR
WDRed PlusWD10EFRX1 TBCMR
WDBlackWD1002FAEX1 TBCMR
WDBlueWD10EZEX1 TBCMR
WDBlueWD10EZRZ1 TBCMR
WDBlueWD10JUCT1 TBCMR
WDBlueWD10JPCX1 TBSMR
WDBlueWD5000AAKX500 GBCMR
WDBlackWD5000LPLX500 GBCMR

Summary by brand

Seagate

  • CMR (the safe choice for RAID/NAS): IronWolf 1–8 TB (ST1000VN002 to ST8000VN004), the entire IronWolf Pro line, Exos (X16, X18, X20, X22), Constellation, older Barracuda 7200 rpm without „DM008".
  • SMR (careful!): Barracuda Compute 2.5” 1–5 TB (ST1000LM048, ST2000LM007, ST5000LM000), ST2000DM006/008, the Archive HDD line, some Skyhawk models (ST8000VX002, ST6000VX010, ST4000VX013, ST4000VX005).

Western Digital

  • CMR: WD Red Plus, WD Red Pro, the entire WD Ultrastar DC HC line, WD Purple Pro, WD Gold, WD Black (except the external NMZW/NPZZ).
  • SMR (careful!): WD Red without „Plus" 2–6 TB (WD20EFAX, WD40EFAX, WD60EFAX), external MyBook/Elements in some capacities (4 TB, 5 TB), WD Blue 2.5” WD10JPCX and the newer 4 TB WD40NPZZ/WD40NMZW.
  • Upgrade path: If you have a WD Red without „Plus" in your NAS — urgently consider replacing it with a WD Red Plus or WD Red Pro.

Toshiba

  • CMR: N300 (all capacities), the MG07 / MG08 / MG09 lines, X300, P300 up to 3 TB, MQ04 (except the L200 1 TB).
  • SMR (careful!): P300 Desktop 4–6 TB (HDWD240UZSVA, HDWD260UZSVA, HDWD260EZSTA), Canvio external (HDTB440, HDTX140), Laptop L200 1 TB.

HGST / Samsung (historical, no longer manufactured)

  • Reliably CMR: HGST Deskstar 5K4000 and 7K4000, HGST Ultrastar HUS724xxx, Samsung Spinpoint F3. You can still find these drives in second-hand shops — for RAID they are safe units.

What to do when I have an SMR drive and my RAID rebuild won’t work

Options from the smallest to the biggest route:

  1. Try the rebuild with lower priority / load. Synology: set „Low priority" in Storage Manager. Linux mdraid: enable the write-intent bitmap (mdadm --grow --bitmap=internal /dev/md0), the bitmap shortens the next resync to minutes. ZFS: zpool set sync=disabled, but careful — there is a risk of losing in-flight transactions during a power outage.
  2. Leave the drive overnight with no further load. After finishing a large write batch, an SMR drive does internal garbage collection (it rewrites the data from the CMR cache into the shingled area). If you do not load it for 6–12 hours, the cache empties and the next rebuild runs faster.
  3. Add another (CMR) drive as a spare, turn the SMR drive into an offline backup, build the array again. Often the fastest solution — if you have a free slot and the money for a replacement.
  4. If the array has crashed completely — call us, we read the drives sector-level (PC-3000, DeepSpar) outside the NAS box, create a RAW image from each unit and reconstruct the array offline. An SMR drive with no pressure in the array can read everything, just slowly — and we have the time.
A Western Digital Blue 2.5-inch hard drive with the cover closed on a black mat, lit by a ring light during the intake inspection
A WD Blue 2.5" drive during the intake inspection. Before we open the drive, we identify the model, verify the symptoms and choose the diagnostic procedure. With SMR drives from a failed array this phase is crucial — we know that hours of slow reading outside the NAS are ahead of us.

FAQ — the most common questions about CMR and SMR

I have a WD Red 4 TB at home in a NAS. How do I tell whether it is SMR? Look at the drive label and find the exact model. WD40EFAX = SMR, WD40EFRX = CMR (older Red), WD40EFPX or WD40EFZX = CMR Plus. If the label does not show a specific model code, you will find it in Synology DSM in Storage Manager, in smartctl -a /dev/sda or on the sticker on the underside of the drive.

I am buying a new drive for a NAS. What is the safest choice today (2026)? For a 4-bay home NAS we recommend Seagate IronWolf (not Pro) in 4–8 TB, WD Red Plus in 4–8 TB or Toshiba N300 in 4–8 TB. All CMR, certified for 24/7 operation, NAS workload rating ~180 TB/year. If you have 8-bay+ and a higher write load, go for IronWolf Pro / Red Pro / N300 — higher price, higher MTBF, 7200 rpm.

What if I just need to create an archival backup? Is SMR OK? For a single-drive external backup using the 3-2-1 scheme (you write once a week, you read during recovery), SMR is completely fine and makes sense for the lower price / higher capacity. An external WD Elements or Seagate Expansion (both often SMR inside) are ideal for an archive. Just never turn an external SMR drive into a NAS bay or a RAID member.

I have heard about „host-aware SMR„ and „host-managed SMR". What is the difference? There are three types of SMR. Drive-managed (DM-SMR) — the drive presents itself as a classic HDD, the firmware handles the cache and migration in the background. This is the problem for RAID/NAS, because the OS does not know about the SMR. Host-aware (HA-SMR) — the drive supports both CMR and SMR write commands, the OS can (but does not have to) optimise. Host-managed (HM-SMR) — sequential writing only via ZBC/ZAC commands, used in cloud object storage systems (Ceph, ZNS-aware). Consumer drives are almost all DM-SMR.

Can an SMR drive be „converted" to CMR via firmware? No. SMR is a physically fixed platter geometry — the tracks are already written with the intention of overlapping. It is not a firmware choice, it is physics. The only thing you can do is switch an SMR drive into „archive mode" at the firmware level (sequential writing only), which reduces its usefulness even further.

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