WD Red Plus vs IronWolf 4TB: Which Drive for a Two-Bay NAS?

A first NAS purchase often starts with a capacity target and ends with several nearly identical drive listings. The useful comparison is between exact models: WD Red Plus WD40EFPX and Seagate IronWolf ST4000VN006. A listing that says only “WD Red 4TB” leaves out information that can change the decision more than the price difference.

The specifications below were checked on October 30, 2026. The intended setup is a small NAS holding photos, documents and other PC files, with two drives configured as a mirror. Higher-capacity models and the Pro families have their own specifications; their spindle speeds and service terms do not automatically describe these 4TB drives.

WD40EFPX and ST4000VN006 share CMR, spindle speed and cache size

The figures come from the September 2025 WD Red Plus data sheet and Seagate sheet DS1904.23-2509US. These are manufacturer specifications, rather than results from a head-to-head NAS benchmark.

Two listings for the same capacity and product family can still offer different models. WD also lists WD40EFZZ as a 4TB Red Plus, with a 128MB cache. Check that the quote names WD40EFPX if that is the drive you intend to buy, and compare it with the label when the package arrives.

WD40EFAX uses DM-SMR, which can stall during mirror rebuilds

The WD Red product brief lists WD40EFAX with 4TB capacity, a 256MB cache, a 180TB/year workload rating and a three-year warranty. It also explicitly identifies the drive as device-managed SMR. None of those four matching numbers establishes that it is equivalent to the two CMR candidates.

SMR overlaps adjacent recording tracks to increase density. Rewriting existing data can require additional work to reorganize the overlapping regions. With device-managed SMR, the drive handles that work behind a normal block interface. A short, apparently successful transfer does not establish how it will respond during a much longer reconstruction workload.

OpenZFS recommends CMR for redundant pools and describes severe write-amplification stalls with DM-SMR during workloads including mirror and RAIDZ resilvering. The concern is how the drive handles these workloads; it does not establish that every SMR disk has failed or predict a fixed number of days for a rebuild.

TrueNAS distinguishes the variants: it discourages DM-SMR and host-aware SMR, while host-managed SMR is unsupported because of the zone management it requires. For a new mirror, the practical choice here is straightforward: use one of the supported CMR models.

Older labels need care, too. WD introduced the Red Plus branding in 2020 to identify CMR products within its NAS family. That history does not turn every older drive bearing a plain Red label into an SMR drive. Match the full model to its documentation when evaluating used drives or an existing array.

Gigabit Ethernet limits transfers to less than either drive’s rated maximum

One gigabit per second divided by eight is 125 megabytes per second. That is the theoretical link ceiling before network and file-sharing overhead. A client connected through a single gigabit path cannot receive a sustained 180MB/s or 202MB/s from the NAS through that path.

Dividing 202 by 180 produces a difference of about 12.2%. It describes the ratio between the published maximum numbers, not a measured improvement in your file-copy time. Paying a substantial premium for that ratio is hard to justify when the intended job is large-file transfers over gigabit Ethernet.

A complete 2.5GbE path raises the theoretical ceiling to 312.5MB/s, above both published drive speeds. Actual mirror performance still depends on NAS processing, concurrent activity, file sizes and where the data sits on the disks; it will not necessarily match a single drive’s specification.

Copying a folder containing thousands of small files requires repeated file operations and disk-head movements. That workload differs from streaming one large file, so the shared 256MB cache figure and maximum transfer rates cannot fully predict how responsive applications will feel.

Two 4TB drives provide 4TB of mirrored capacity, so compare pair prices

Two equal-capacity disks in RAID 1 store duplicate data. Two 4TB disks therefore provide a nominal 4TB mirrored capacity, rather than 8TB of independent file space. Expressed in binary units, four trillion bytes are approximately 3.64TiB, before further reservations.

The Synology RAID calculator notes explain that system partitions and filesystem metadata reduce usable capacity further. A lower displayed number is not evidence that a disk went missing. Compare capacity in matching units and include the array layout.

Use the price of both disks when evaluating a premium. For a currency-neutral example, assume one candidate costs 100 units per disk and the other costs 110, with identical delivery and warranty conditions. The mirrored pair costs 200 or 220 units. Dividing by its nominal 4TB gives 50 or 55 units per usable TB. Dividing by the raw 8TB total would understate the cost of the mirrored capacity by half.

These are hypothetical quotes, not current retail prices. Apply the same calculation to local offers to compare their cost per mirrored TB. If 4TB of mirrored capacity is already too small, get quotes for a larger pair before comparing these brands at 4TB.

A two-bay enclosure has no empty bay after this purchase. Future growth then involves replacing drives or changing the storage setup. Allow space for retained versions and new files, rather than sizing the purchase only around the originals already on your PC.

The 180TB/year workload rating, hardware warranty and Rescue coverage

The annual workload rating measures transferred data, including reads and writes. It is not the drive's storage capacity or a lifetime write allowance like an SSD's TBW specification. Matching 180TB/year ratings do not establish which brand will last longer.

For a simple workload example, 300GB read plus 100GB written per day totals approximately 146TB over 365 days. Client traffic is not automatically the same as each disk's traffic: mirroring and internal tasks affect what the disks process. A folder's current size is also not its annual workload.

The hardware warranty covers replacement under its terms; data recovery is a separate service. Seagate lists three years of Rescue for this model, with a footnote that service is unavailable in some countries. If that service influences your purchase, check that the actual product qualifies in your market. You still need a separate copy of your files: recovery service does not provide one, and the limited hardware warranty does not promise to restore their contents.

Choose the cheaper supported pair; at equal prices, consider IronWolf’s Rescue service

For ordinary photo and document storage over gigabit Ethernet, choose the cheaper supported CMR pair when warranty and sales conditions match. Both candidates already satisfy the recording-design requirement. There is little in their shared headline specifications to justify paying extra solely for the brand.

At the same price, choose ST4000VN006 if your purchase qualifies for Rescue and you value that service. Its higher published maximum transfer rate also deserves consideration for large-file work over a faster network. If the enclosure will sit beside your desk, look for noise measurements or reports for the exact drive and case: identical spindle speeds do not predict identical installed noise.

For a replacement in an existing array, follow the enclosure’s replacement capacity requirements and procedure. Use the NAS vendor’s drive list, such as Synology's HDD/SSD compatibility page, to check the exact enclosure and drive combination; CMR alone does not establish compatibility. Once both candidates meet those requirements, compare local service terms and the total price for each pair. Pay more only if the additional benefit, such as eligible Rescue coverage or a higher transfer-rate specification relevant to your workload and network, is worth the extra cost to you.