For a NAS array that must handle sustained writes and disk replacement, choose a documented CMR model. TrueNAS’s hardware guide recommends avoiding SMR drives because of their write and resilver behavior. 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. This guide explains how to use that result in a purchase.
CMR or SMR for NAS: decide by the write workload
CMR means conventional magnetic recording. SMR means shingled magnetic recording, which overlaps tracks to increase recording density. Western Digital’s Zoned Storage documentation explains why updating shingled data can require additional work to preserve neighboring tracks. That extra work matters more to a buyer than the names of the recording methods.
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 OpenZFS documents resilvering as bringing an out-of-date device into synchronization; replacement activity competes with the normal work of the pool.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.
There is no universal “CMR rebuild takes one day, SMR takes one week” conversion. 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 for that use under the same guidance |
| 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 |
| 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.
How to check CMR or SMR before buying a NAS drive
Find the full model identifier on the label, inventory screen or manufacturer’s listing. Match that identifier against the vendor’s own table. Do not substitute the serial number, a retailer’s stock code, or the enclosure model. Those identifiers may refer to different things.
The CMR or SMR lookup 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. If the disk reports extra revision characters, confirm how that identifier maps to the vendor’s published model before drawing a conclusion.
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. 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.
