SMR drives are slower under sustained random write workloads, which makes them unsuitable for most NAS use cases. The controversy around SMR became public knowledge in 2020 when Western Digital was found to have shipped SMR drives in its NAS-targeted WD Red product line without clearly disclosing the recording technology. The issue matters because SMR drives behave acceptably as a secondary backup destination or cold archive, but cause serious problems in RAID arrays: elevated rebuild times, timeout errors, array degradation, and in some cases, RAID controllers dropping the drive from the pool entirely during a rebuild.
In short: For any NAS running RAID, use CMR drives only. For conventional RAID/ZFS and active multi-user workloads, CMR is the safer choice. Device-managed SMR suitability depends on the exact drive, workload, storage stack and NAS vendor compatibility list. When in doubt, buy WD Red Plus, WD Red Pro, Seagate IronWolf, or Seagate IronWolf Pro - all are CMR.
What SMR and CMR Mean
CMR (Conventional Magnetic Recording) writes data in non-overlapping tracks. Each track is written independently and can be updated without affecting adjacent tracks. This is the traditional recording method that hard drives have used for decades. CMR handles random writes efficiently because overwriting any track does not require rewriting neighbouring tracks.
SMR (Shingled Magnetic Recording) overlaps tracks like roof shingles, with each track partially written over the previous one. This increases areal density and therefore capacity per platter, allowing larger capacity drives at the same or lower cost. The downside: updating any data in the middle of a shingle band requires rewriting all the data in that band. To manage this, SMR drives use an internal write cache (called a persistent media cache or PMC) to buffer incoming writes, then batch-reorganise the data in the background during idle periods. Under sustained random write workloads, this cache fills faster than it can be cleared, causing severe write performance degradation as the drive stalls to reorganise its internal data structures.
Why SMR Fails in RAID Rebuilds
A RAID rebuild reads and writes large volumes of data sequentially, but the actual I/O pattern at the drive level is mixed sequential and random depending on the RAID level and file system. An SMR drive under this workload fills its write cache and stalls. Some SMR drives and RAID/ZFS implementations have exhibited long stalls or errors during rebuilds; whether a drive is dropped depends on its firmware, workload and the storage stack's timeout and error-handling behaviour.
Dropping a second drive from an already-degraded array causes total array failure in RAID 5. Even in RAID 6, dropping a second drive during a rebuild leaves the array exposed and produces errors. This was not merely theoretical: users reported problems and independent tests reproduced severe rebuild degradation with WD Red DM-SMR drives, particularly under ZFS.
Even if the drive does not get dropped, an SMR drive in a rebuild can extend rebuild time from hours to days. An SMR drive can substantially extend a rebuild, but the size of the penalty varies with the drive model, RAID implementation, array state and workload. During that extended rebuild window, the risk of a second drive failure compounds.
2026 Drive Reference: CMR vs SMR
The following covers the current NAS-targeted drive lines from the major brands available in Australia. All drives targeted specifically at NAS use in the Plus, Pro, or N-series tiers from these manufacturers are CMR. The problem historically arose from consumer-tier drives (WD Blue, some WD Red base models) being used in NAS devices where buyers assumed NAS-brand meant NAS-suitable recording technology.
CMR vs SMR status for NAS-relevant drive lines (2026)
| Recording type | NAS suitable | Notes | |
|---|---|---|---|
| Seagate IronWolf (all capacities) | CMR | Yes | Standard NAS line. 3-year warranty. 180TB/yr workload. |
| Seagate IronWolf Pro (all current capacities, up to 32TB) | CMR | Yes | Pro NAS line. 5-year warranty. Up to 550TB/yr workload. |
| Seagate BarraCuda | Recording technology varies | No - desktop drive | Seagate currently lists 4TB and 8TB BarraCuda as SMR and 12TB-24TB as CMR; verify the exact model. |
| Seagate BarraCuda (8TB) | SMR | No | Desktop line. Seagate currently lists its 12TB-24TB BarraCuda capacities as CMR. |
| WD Red Plus (all current capacities, 2TB-12TB) | CMR | Yes | WD's explicit CMR NAS line, introduced following the 2020 controversy. |
| WD Red Pro (all current capacities, 2TB-26TB) | CMR | Yes | Enterprise NAS line. 5-year warranty. 550TB/yr workload. |
| WD Red (base, 2TB-6TB) | SMR | Conditional | Current light-workload personal/home NAS line; verify compatibility and avoid for ZFS or sustained write-intensive workloads. |
| WD Blue (1TB-6TB) | Model-specific | No - desktop drive | Current 2TB, 3TB and 4TB offerings include both CMR and SMR model variants; verify the exact model. |
| WD Blue (8TB-12TB) | CMR | No - desktop drive | Current WD Blue models in these capacities are CMR, but remain desktop products. |
| Toshiba N300 (all capacities) | CMR | Yes | Toshiba's dedicated NAS line. Available through some AU retailers. |
| Toshiba X300 (all capacities) | CMR | No - desktop/performance | CMR but performance desktop drive. Can work in NAS but not optimal. |
The safe shortlist for AU buyers: Seagate IronWolf, Seagate IronWolf Pro, WD Red Plus, WD Red Pro, Toshiba N300. Each is a CMR, NAS-oriented drive family, but buyers must still check the exact drive model, capacity and firmware against their NAS vendor's compatibility list. Avoid base WD Red (pre-2022 stock may still appear on marketplaces), desktop drives at any capacity, and any drive not explicitly marketed for NAS use.
How to Check if a Drive Is SMR or CMR
Model number lookup. The most reliable method. Search the exact model number (from the drive label or SMART data) against the manufacturer's product page or the regularly updated community-maintained SMR/CMR drive lists on forums such as r/datahoarder and the TrueNAS community wiki. Community-maintained lists can help identify older or unusual retail variants, but verify their entries against the manufacturer's current data sheet wherever possible.
Manufacturer specification pages. Seagate and Western Digital both now disclose recording technology on product specification pages following the 2020 controversy. Look for "CMR" or "Conventional Magnetic Recording" in the drive's tech specs. If the spec sheet does not mention recording technology at all, treat the drive as unverified and cross-reference against community lists.
CrystalDiskInfo (Windows). CrystalDiskInfo can display the drive's model number, rotation rate and SMART data, but those fields do not establish whether it uses SMR or CMR; use the exact model number for verification. It is not definitive for all models but useful for drives already installed.
Sustained write test. A sufficiently large, properly preconditioned sustained-write test may reveal SMR behaviour, but it is not definitive and should not replace exact-model verification. This speed cliff is clearly visible in a disk benchmark like CrystalDiskMark with queue depth 1, sequential writes over several passes.
What to Do If You Already Have SMR Drives in a NAS
If your NAS has SMR drives currently and the array is running without problems, the practical options depend on how the NAS is being used.
If the NAS is a backup destination only (sequential write, infrequent random access): SMR drives can remain in service. Backup workloads are largely sequential and infrequent enough that the write cache does not fill. Monitor SMART data and rebuild health, but immediate replacement is not required.
If the NAS is an active file server, Plex server, or multi-user system: Plan replacement at next available opportunity. The random I/O patterns of active file serving will degrade SMR performance over time and create rebuild risk if the array needs to rebuild. Follow the NAS vendor's documented replacement procedure for the exact RAID or storage-pool configuration, and ensure a current backup before replacing drives.
If you have an existing RAID array you need to rebuild: Rebuild speed on SMR drives will be significantly slower than CMR. Rebuild times may be significantly longer, but the penalty varies widely with the drive, array, file system, occupancy and concurrent workload. Do not run any other I/O-intensive workloads during the rebuild. Ensure you have a current backup before starting any rebuild with SMR drives.
Australian Buyers: What You Need to Know
Marketplace stock. Base WD Red 2TB-6TB SMR models remain listed by Western Digital and continue to appear in Australian retail and marketplace listings. The model numbers to be wary of are WD20EFAX, WD30EFAX, WD40EFAX, and WD60EFAX (the 2TB, 3TB, 4TB, and 6TB base WD Red SMR variants). If you see WD Red drives (not Red Plus) at unusually low prices, check the exact model number before purchasing. Current WD Red Plus models use several suffixes, including EFPX, EFZX, EFZZ, EFGX and EFBX, and are listed as CMR; verify the complete model number against the current data sheet.
Drive pricing and value. With NAS-grade drive prices elevated in 2026, the price gap between consumer desktop drives and NAS-grade CMR drives is a real consideration. Seagate IronWolf and WD Red Plus are priced at a premium over desktop BarraCuda and WD Blue drives. The premium is justified: NAS drives include vibration sensors, TLER (Time-Limited Error Recovery) for RAID controller compatibility, and the CMR recording technology required for reliable RAID operation. Using desktop drives to save money risks array stability and creates a rebuild risk that is not worth the saving for a device holding important data.
Australian Consumer Law and drive warranties. ACL consumer guarantees are separate from voluntary manufacturer warranties. For an ACL remedy, contact the seller first; the seller cannot refuse assistance by directing you to the manufacturer. A manufacturer-warranty claim may also be made with the warranty provider under its terms. Consider keeping one spare NAS-grade CMR drive on hand for business-critical NAS setups to avoid running in a degraded state during a warranty exchange.
Related reading: our NAS buyer's guide.
Use our free NAS Sizing Wizard to get a personalised NAS recommendation.
Related reading: our NAS hard drive guide.
Are all WD Red drives SMR?
No. The base WD Red line (2TB-6TB) sold between 2019-2022 included SMR models. Western Digital created the WD Red Plus line specifically to offer a clearly labelled CMR NAS drive following the 2020 controversy. Current WD Red Plus and WD Red Pro drives are all CMR. If you are buying new drives for a NAS, buy WD Red Plus or WD Red Pro, not base WD Red.
Can SMR drives be used in any NAS configuration safely?
SMR drives can be used in a NAS as a pure backup destination where the workload is primarily sequential writes (backing up from a PC, Synology Hyper Backup destination, Time Machine target) with no RAID array. In this specific use case, the write cache characteristics of SMR are less likely to cause problems because the I/O pattern is sequential and infrequent. SMR should be avoided for ZFS, sustained random-write workloads and RAID configurations where the exact drive is not approved by the NAS vendor. Light-workload suitability remains model- and platform-specific.
Does Synology or QNAP block SMR drives?
Synology maintains a compatibility list that includes specific tested drive models. Synology and QNAP compatibility is model-specific. Check the exact NAS and drive combination on the applicable compatibility list; an unlisted or incompatible drive may receive warnings, functional restrictions or limited support. While both NAS platforms will physically accept SMR drives, running them in RAID configurations on an incompatible list may affect support coverage for any issues that arise during operation.
Is Seagate BarraCuda safe for NAS?
No, for two reasons. First, BarraCuda is a desktop drive without TLER (Time-Limited Error Recovery) for RAID controller compatibility or the vibration compensation sensors included in NAS-grade drives. Second, some BarraCuda models at higher capacities use SMR recording. For a NAS, use Seagate IronWolf (CMR, NAS-specific) rather than BarraCuda regardless of recording technology.
What is TLER and why does it matter for NAS drives?
TLER (Time-Limited Error Recovery) is a setting that limits how long a drive will spend attempting to recover from a read error before reporting the error to the RAID controller. Desktop drives are configured with long recovery timeouts because they are not managed by RAID controllers. RAID-oriented drives use bounded error-recovery behaviour to reduce the risk of long recovery attempts triggering host timeouts. Western Digital states a seven-second limit for its RAID drives, but host and controller timeout values vary by implementation.
How do I find the model number of a drive already in my NAS?
On Synology DSM: open Storage Manager, select the HDD/SSD tab, click on any drive to see its model number and serial number. On QNAP QTS: open Storage and Snapshots, go to Disks/VJBOD, select any drive. Both platforms also show this information under their respective SMART tools. The model number can then be searched against manufacturer spec pages or community SMR/CMR lists to confirm recording technology.
Once you have the right drives installed, understanding how failure probability scales with age and capacity helps you plan replacement cycles. The drive failure probability guide covers MTBF, AFR, and what the numbers mean in practice.