Buy CMR drives for a NAS. Every disk in a RAID, SHR or ZFS array should be a documented CMR model, such as WD Red Plus, WD Red Pro, Seagate IronWolf or Toshiba N300, because drive-managed SMR disks slow sharply under sustained writes and rebuilds. Keep SMR for a standalone backup disk that is written once and rarely rewritten.
The TrueNAS hardware guide recommends avoiding SMR drives: they offer more density than CMR equivalents but slower write and overwrite performance, and drive-managed models bring longer, less reliable resilvers. That is a workload decision. Two disks with the same capacity and SATA interface can behave differently when repeatedly rewritten.
Start with the exact model in the CMR/SMR drive lookup. It links each listed result to a manufacturer table and leaves unlisted models unknown. Then check appliance compatibility and the drive’s workload specifications separately.
CMR vs SMR: what is the difference?
CMR (conventional magnetic recording) leaves gaps between recording tracks, so a drive can overwrite any sector in place. SMR (shingled magnetic recording) removes those gaps by overlapping each new track onto part of the previous one. That packs in more data, but modifying one sector means rewriting the whole band of tracks it sits in.
Both definitions come from Western Digital’s Zoned Storage documentation. The extra rewrite work matters more to a buyer than the names of the recording methods, and it has three practical consequences:
- The penalty follows rewrites. Seagate’s CMR/SMR list says SMR fits sequential or easily sequentialized workloads such as media repositories, backup tiers and compliance archives. A NAS array that keeps modifying files and must rebuild onto replacement disks is the opposite workload.
- Drive-managed SMR looks like any other disk to the NAS. The firmware enforces the sequential-write rule behind a backwards-compatible interface, so the NAS formats and uses it normally. ServeTheHome describes how such drives stage writes in a CMR cache area and pay the full penalty once sustained writes exhaust it.
- SMR still leads on capacity. Seagate’s list, checked in October 2026, shows SMR Exos drives at 32TB, 36TB and 44TB, while its CMR drives stop at 24TB, or 32TB with Mozaic HAMR.
CMR or SMR for NAS: decide by the write workload
For a frequently updated file server, a parity array, or storage for virtual machine disks, prefer CMR. The recommendation is especially relevant when a replacement disk must accept reconstructed data while the rest of the NAS stays busy. CMR avoids the shingled-track rewrite constraint, although it does not guarantee any particular transfer rate.
For infrequently rewritten storage, evaluate the actual device and the time available to finish a copy. A drive-managed SMR disk presents an ordinary block interface, while host-managed SMR requires software that understands zoned writes. Do not treat those two kinds of SMR as interchangeable when shopping for a conventional NAS. A product intended for a managed archive is not automatically a drop-in replacement in a home array.
CMR drives for NAS rebuilds and resilvers
A failed-drive replacement is a reason to check recording technology before an emergency. OpenZFS documents resilvering as bringing an out-of-date device into synchronization; replacement activity competes with the normal work of the pool. The amount of data, other activity, layout and drive behavior all affect how long it takes.
SMR can add a write-workload constraint during that process. TrueNAS explicitly warns about SMR write and resilver performance. Avoid treating a short sequential copy as proof that a drive will be suitable for a long replacement operation. A favorable burst result does not establish sustained performance under the array’s workload.
The best-known published comparison shows how far apart the two can land. In ServeTheHome’s May 2020 RAIDZ resilver test, all three CMR drives finished in under 17 hours, while the SMR WD40EFAX took nearly 230 hours, about nine and a half days. QNAP’s support FAQ applies the same warning to all of its NAS series, naming WD20EFAX and WD60EFAX as SMR models that affect RAID rebuild performance.
Treat those figures as one test, not a conversion factor: there is no universal “CMR rebuild takes one day, SMR takes one week” rule. Keep backups current and select the replacement against the NAS vendor’s supported-drive guidance. If the machine is already slow, use the NAS hardware transfer diagnosis to separate drive behavior from a network or controller ceiling.
CMR vs SMR drives: a NAS buying comparison
| Buying question | CMR | Drive-managed SMR |
|---|---|---|
| How are tracks arranged? | Conventional non-overlapping recording | Overlapping shingled tracks |
| What changes on a rewrite? | No shingled-band preservation step | The drive manages the extra work of preserving affected data |
| Suitable starting point for a busy NAS array? | Preferred in the cited TrueNAS hardware guidance | Avoid; TrueNAS does not recommend it, and host-managed SMR does not work at all |
| Published 4TB RAIDZ resilver (ServeTheHome, 2020) | Under 17 hours | Nearly 230 hours |
| Does a short fast copy settle the decision? | No; it describes only that workload | No; it may miss sustained rewrite constraints |
| Can capacity or RPM identify it? | Use a model-specific disclosure | Use a model-specific disclosure |
| NAS-branded examples | WD Red Plus, WD Red Pro, IronWolf, IronWolf Pro, Toshiba N300 | Plain 2TB to 6TB WD Red (2020 disclosure) |
| Largest Seagate capacity (October 2026) | 24TB, or 32TB with HAMR | 44TB Exos |
| Does the label certify appliance compatibility? | No; check the NAS compatibility list | No; check the NAS compatibility list |
Read the table as selection guidance, not performance measurements. The drive’s firmware, interface, workload and the storage platform still matter. Recording technology answers one question about the disk, rather than replacing its entire specification sheet.
Which NAS drives are CMR?
WD Red Plus, WD Red Pro, Seagate IronWolf, IronWolf Pro and Toshiba N300 are all listed as CMR by their manufacturers. The SMR traps are the plain 2TB to 6TB WD Red drives covered by WD’s 2020 disclosure, Seagate’s 3.5-inch BarraCuda 4TB and 8TB, and Toshiba’s S300 surveillance line.
| NAS drive family (vendor pages, checked October 2026) | Recording | Capacities listed | Workload rating | Bay rating |
|---|---|---|---|---|
| WD Red Plus | CMR | Up to 12TB | 180TB/year | Up to 8 bays |
| WD Red Pro | CMR | Up to 26TB | 550TB/year | Unlimited |
| Seagate IronWolf | CMR | 2TB to 16TB | 180TB/year | Multi-bay; no count on the product page |
| Seagate IronWolf Pro | CMR (HAMR at 28TB to 32TB) | 2TB to 32TB | 550TB/year (24TB model) | No count in the cited pages |
| Toshiba N300 | CMR | 4TB to 22TB | 180TB/year | Up to 12 bays |
Warranty tracks the workload tier. WD lists three years for Red Plus and five for Red Pro. Toshiba lists three years for the N300 and five for the N300 Pro, which its lineup table rates at 550TB/year and up to 24 bays. Seagate’s 24TB IronWolf Pro launch in January 2024 quoted 550TB/year, 2.5 million hours MTBF and a five-year warranty at a $649 MSRP.
Three details catch buyers out. A NAS-sounding name is not enough: WD’s 2020 post lists its then-current 2TB, 3TB, 4TB and 6TB WD Red drives as drive-managed SMR, while ServeTheHome’s CMR comparison drive was the older WD40EFRX, also a 4TB WD Red. A family name is not enough either: Seagate lists 3.5-inch BarraCuda 4TB and 8TB as SMR but 12TB to 24TB BarraCuda as CMR. And a bay rating does not imply CMR, since Toshiba rates its drive-managed SMR S300 for up to 8 bays.
CMR alone also does not make a desktop drive a NAS drive. The TrueNAS guide notes that consumer drives “are still not designed or warrantied for continuous operation or use in RAID groups,” so a CMR BarraCuda answers only the recording question.
How to check CMR or SMR before buying a NAS drive
- Get the full model number. Read it from the label, the manufacturer’s listing, or the NAS storage screen for an installed disk. Do not substitute the serial number, a retailer’s stock code, or the enclosure model; those identifiers refer to different things.
- Run it through the CMR or SMR lookup. It includes selected WD Red, Red Plus, Seagate BarraCuda and IronWolf models. A partial query only filters possible matches. An exact match displays the documented recording method and a link to the table; a missing entry remains unclassified.
- Confirm with the manufacturer. Use Seagate’s list for family and capacity, WD’s product briefs for model numbers, and Toshiba’s lineup table for each series. If the disk reports extra revision characters, confirm how that identifier maps to the vendor’s published model before drawing a conclusion.
- Check the NAS vendor’s compatibility list. QNAP’s drive-mixing FAQ says to check the list for your NAS model before purchasing and to use identical drives within each RAID group.
- Match the ratings to the chassis. A 12-bay enclosure exceeds Red Plus’s 8-bay rating, and a busy multi-user array is the case the 550TB/year tiers are written for.
Western Digital’s April 2020 disclosure lists WD40EFAX as SMR. Its March 2025 Red Plus brief lists WD40EFPX as CMR. Both are 4 TB models. The different model letters matter; “4 TB WD Red” is insufficient to settle the question.
Seagate’s recording-technology list is useful for checking families and capacities, but use a model-specific datasheet to confirm a particular purchase. The lookup cites those model tables directly. For secondhand stock, the dated specification matching the actual model is more useful than the current marketing page for a similarly named family.
Choosing between two CMR drives for NAS
Once recording technology is settled, compare the specifications that remain. WD’s cited Red Plus brief lists both WD120EFGX and WD120EFBX as 12 TB CMR drives, but their stated internal transfer rates, cache sizes and power requirements differ. A common family name does not make every model electrically or mechanically identical.
For a chassis decision, take startup-current figures from the exact model’s table. For appliance use, consult the supported-drive list. For expected workload, read the manufacturer’s definition of its workload rating. None of those questions is answered by a CMR label alone.
Use the NAS buying guide for bay count, controller connectivity and memory, then the NAS Drive Bay and PSU Calculator for an initial bay and startup-power estimate.
Browse related NAS guides to connect drive selection with chassis planning and transfer troubleshooting.
What if an SMR drive is already in the NAS?
Identify the model and review its suitability for the array before making changes; the shingled magnetic recording guide shows how to read the model and zone type from a Linux shell. A recording-method lookup is not a health diagnosis, and finding SMR does not establish that the disk has failed. Check backups, current array health and the platform’s documented replacement procedure.
If the model is unsuitable for the planned workload, schedule a supported replacement. Do not pull several members merely because they appear on an SMR disclosure table. Preserving the array’s available redundancy matters while any replacement completes.
For a drive kept outside an array, consider how often its contents are rewritten and whether the backup window accommodates its sustained performance. It can remain one backup destination if it meets that workload, but a completed copy and a verified restore are more meaningful than an assumption based on the product label.
Is CMR always faster than SMR?
No single speed ordering covers every model and workload. The NAS recommendation concerns rewrite behavior and replacement workloads, not a claim that every CMR read is faster than every SMR read. Use published specifications for the exact model and keep the expected workload in view. For an array purchase, documented CMR and appliance compatibility are the starting checks.
FAQ
what does cmr mean on a hard drive
CMR stands for conventional magnetic recording. A CMR hard drive writes its tracks side by side with a gap between them, so it can overwrite any sector in place without disturbing neighboring tracks. Manufacturers label NAS lines such as WD Red Plus, Seagate IronWolf and Toshiba N300 as CMR to separate them from shingled SMR models.
is wd red cmr or smr
The 2TB to 6TB plain WD Red models Western Digital disclosed in 2020, such as WD20EFAX, WD40EFAX and WD60EFAX, are drive-managed SMR. WD Red Plus and WD Red Pro are CMR, as was the older WD40EFRX. Since a 4TB WD Red and a 4TB WD Red Plus look alike, check the model number before buying.
are seagate ironwolf drives cmr or smr
Seagate IronWolf and IronWolf Pro drives are CMR at every capacity on Seagate’s recording-technology list, with the 28TB to 32TB IronWolf Pro models using HAMR-based CMR. Seagate’s SMR models sit elsewhere: 3.5-inch BarraCuda 4TB and 8TB, 2.5-inch BarraCuda and FireCuda, the 8TB Archive drive and the largest Exos capacities.
can you mix cmr and smr drives in a nas
You can, but keep them in separate storage pools. QNAP advises identical drives (same brand, model, capacity and rotation speed) within each RAID group and says a group runs at the speed of its slowest drive, so one SMR member slows a CMR array’s heavy writes. An SMR disk on its own single-disk volume avoids that.
