HAMR and UltraSMR both push hard drives past 30TB, and ZFS treats them in opposite ways. A 32TB HAMR drive such as the Seagate IronWolf Pro reports itself as an ordinary CMR disk, so a ZFS pool uses it like any other drive. The UltraSMR drives Western Digital documents, the Ultrastar HC680 and HC690, are host-managed SMR, and OpenZFS has no code to write to one.
We checked the datasheets, the OpenZFS documentation and every Amazon US listing on October 21, 2026, then rechecked the linked listings on November 1, 2026, after the WD Red Pro 26TB sold out. This is a spec comparison, not a lab test: no 30TB+ drive passed through our bench, and the resilver figures below are computed from datasheet speeds. For the basics of shingled recording and the WD Red story, start with our CMR vs SMR guide.
HAMR vs UltraSMR at a glance
The column that matters for a ZFS pool is what the drive presents to the host. Every drive in this generation runs at roughly the same sequential speed, while recording type and availability split them cleanly:
| Drive | Recording | Host sees | Capacity | Max sustained | Who can buy it |
|---|---|---|---|---|---|
| Seagate IronWolf Pro 32TB | HAMR, 10 platters (StorageReview) | CMR | 28 to 32TB | 285 MB/s | Retail, thin stock |
| Seagate Exos 32TB, Exos M 30TB | HAMR, Mozaic 3+ | CMR | 28 to 32TB | 270 to 285 MB/s | Retail; Amazon has third-party and renewed stock only |
| Seagate Mozaic 4+ | HAMR, over 4TB a platter | Not published | Up to 44TB | About 300 MB/s (StorageReview) | Two cloud providers at launch |
| WD Ultrastar DC HC680 | UltraSMR on EAMR | Host-managed SMR | 26 to 28TB | 265 MB/s | OEM and hyperscale |
| WD Ultrastar DC HC690 | UltraSMR on ePMR, 11 disks | Host-managed SMR | 30 to 32TB | 269 MB/s | Sales inquiry only |
| WD 40TB UltraSMR | UltraSMR on ePMR | SMR | 40TB | Not published | Hyperscale |
| Toshiba M12 | FC-MAMR plus SMR, 11 glass disks | Host-managed SMR | 30 to 34TB | 282 MiB/s | Samples only |
Two patterns fall out of the table. Every 30TB+ drive on sale new in US retail is HAMR CMR from Seagate. Every SMR drive at 26TB and up is sold to data centers, and each one with a published interface is host-managed. Sequential speed is flat at 260 to 300 MB/s across the whole generation, so the time to fill a drive grows almost in step with its capacity.
Where the figures come from
Each number in this comparison traces to one of four kinds of source, and we kept them apart:
- Datasheets and manuals: capacity, sustained rate, workload rating and zone layout, from Seagate, WD and Toshiba documents dated 2024 to 2026.
- Reviews and press: platter counts, the 44TB speed and the WD 40TB ship date, each attributed to the review or press release that published it.
- OpenZFS sources: the Hardware page, the zpool-replace man page, the Linux vdev source and the issue tracker.
- Amazon listings: price, seller and stock, pulled through the Amazon API and checked on the product page the same day.
Where a maker has not published a figure, the table says so instead of filling the gap. Seagate has not said whether its 36TB and 44TB drives are CMR or SMR, and WD has published no model name or speed for its 40TB drive.
Step 1: Know what HAMR changes
HAMR, heat-assisted magnetic recording, heats a tiny spot of the platter with a laser for each write, so smaller magnetic grains can hold a bit. The Seagate Mozaic 3+ platform passed 3TB per platter. That gives 30TB on ten platters and 32TB on the current Exos and IronWolf Pro. Mozaic 4+ passes 4TB per platter and reaches up to 44TB per drive, and at its March 2026 launch Seagate shipped it to two hyperscale cloud providers only.
HAMR is a recording method, not a track layout, so it can be paired with CMR or SMR. The Seagate datasheets list the retail Exos 32TB and Exos M 30TB as CMR, and its January 2026 launch said 32TB CMR capacity across Exos, SkyHawk AI and IronWolf Pro. The IronWolf Pro datasheet itself says only CMR; the HAMR platters come from the StorageReview review, which counted ten of them.
A HAMR CMR drive presents as a normal, non-zoned SATA disk. ZFS, mdadm and every NAS OS treat it like any 24TB drive. We found no OpenZFS issue or document that handles HAMR differently. Seagate also shipped a 32TB SMR Exos M to hyperscalers, which is why some guides still label every 32TB Exos as SMR. The retail part, ST32000NM004K, is CMR.
Step 2: Know why UltraSMR stays in data centers
UltraSMR is the shingled format from Western Digital. It overlaps tracks like roof shingles and adds large block encoding with stronger error correction to raise track density further. On WD’s 11-disk platform, CMR tops out at 26TB in the WD Gold and Ultrastar HC590, while the UltraSMR HC690 reaches 32TB, about 23 percent more capacity per drive.
Both shipping Ultrastar models are host-managed. The HC680 SATA specification lists 256 MiB zones, 1 percent of capacity as conventional zones and the ZAC-2 command set. A 28TB HC680 carries 1,044 conventional zones and 103,268 sequential-write-required zones, and the host must write each of those in order. The WD store shows the HC690 with a sales inquiry form and no cart, and Amazon US has no listing for either model.
The 40TB drive follows the same path. WD started shipping its 40TB ePMR platform in the quarter that ended July 3, 2026, as UltraSMR to hyperscale customers, StorageReview reports. The WD roadmap puts a 44TB HAMR drive in the first half of 2027 and ePMR at 60TB in 2028.
Toshiba M12: microwave-assisted SMR
The Toshiba M12 series takes a third route: flux-control microwave-assisted recording (FC-MAMR) with SMR on 11 glass disks. It holds 30 to 34TB at 282 MiB/s, with a 550 TB per year workload rating. Toshiba states the SMR models are host-managed, and they were sampling to data center customers from March 2026. A CMR version at up to 28TB was due to sample in the third quarter, with no retail date.
Step 3: Separate the two ways SMR breaks ZFS
SMR fails in ZFS in two unrelated ways, and only one of them is the 2020 WD Red problem. The difference is where the shingle logic lives: in the drive firmware, or in the host software.
The figure shows why one rule does not cover both. ZFS can write to a drive-managed disk but slows to a crawl when the firmware falls behind, and it has no way to write to a host-managed disk at all.
Drive-managed SMR: stalls and IDNF errors during resilver
A drive-managed SMR disk hides its zones behind a normal block interface and rewrites shingled bands in the background. A resilver writes for hours without idle time, so the firmware falls behind. In the ServeTheHome test, three CMR drives finished a RAIDZ resilver in under 17 hours. The SMR WD Red took nearly 230 hours, about 13.5 times longer.
OpenZFS added an SMR section to its Hardware documentation in October 2026. It says drive-managed SMR “can trigger severe write-amplification stalls during RAIDZ/mirror resilver, scrub, and zfs send/receive workloads” and that these drives are not recommended for ZFS pools. In the iXsystems lab the failure went further: WD40EFAX drives on firmware 82.00A82 returned IDNF errors under resilver load, and ZFS faulted them as failed disks.
Host-managed SMR: ZFS cannot use the drive at all
A host-managed drive refuses random writes to its sequential zones, so the filesystem has to write each zone from start to end. OpenZFS has no code for that. The Linux disk layer in the OpenZFS source has no zone handling, no release note mentions zoned devices, and the issue tracker shows no feature work on it.
In a 2026 feature-request thread, TrueNAS staff answered a question about the HC620 with “No”, adding that HM-SMR drives will not work with TrueNAS or ZFS. The drive may not even show up: StorageReview found a host-managed Exos X26z invisible behind a Supermicro SAS backplane and visible only on a motherboard SATA port. We found no host-managed SMR support documented by Unraid, Synology or QNAP either.
Step 4: Decide whether a host-managed drive has any home use
Used and recertified host-managed drives turn up cheap on eBay. Linux can drive one, but only safely as a single disk, through the zoned layers listed by zonedstorage.io:
- Btrfs zoned mode: stable in the single and DUP profiles; RAID1 on zoned disks needs the experimental raid-stripe-tree, and RAID5 and RAID6 are not implemented (btrfs docs).
- XFS zoned: available since kernel 6.15, which logs the feature as experimental at mount.
- dm-zoned: a device-mapper target that turns the drive back into a random-write device, so ext4 or XFS can sit on top.
- F2FS: supports zoned drives natively and was designed for flash storage.
- zonefs: exposes each zone as a file, closer to raw block access than a POSIX filesystem.
None of these gives you a stable redundant array, and zonedstorage.io warns that SAS HBAs may not support the drive at all. StorageReview measured 2 random 4K write IOPS on its Exos X26z before the test failed. A host-managed drive fits one job at home: a single append-only backup or archive target on direct SATA. Keep it out of any ZFS pool.
Step 5: Plan for resilver time at 32TB
Since a HAMR CMR drive behaves like any other disk in ZFS, its real cost is the length of a resilver. Nobody has published a measured ZFS resilver on a 30TB+ drive, so this table divides capacity by the best sequential rate on each datasheet. That is the floor for a completely full drive:
| Drive | Capacity | Datasheet max rate | Floor for a full drive |
|---|---|---|---|
| WD Red Pro 24TB (CMR) | 24TB | 287 MB/s | 23.2 hours |
| Seagate Exos M 28TB (HAMR CMR) | 28TB | 270 MB/s | 28.8 hours |
| Seagate Exos M 30TB (HAMR CMR) | 30TB | 275 MB/s | 30.3 hours |
| Seagate IronWolf Pro or Exos 32TB (HAMR CMR) | 32TB | 285 MB/s | 31.2 hours |
| WD HC690 32TB (host-managed, for scale) | 32TB | 269 MB/s | 33.0 hours |
| 44TB at about 300 MB/s (StorageReview figure) | 44TB | 300 MB/s | 40.7 hours |
ZFS resilvers only allocated blocks, so scale the floor by how full the pool is: a full 32TB drive needs at least 31 hours, a half-full one about 16. A RAIDZ resilver also walks the block tree, which the OpenZFS dRAID guide describes as a random read workload, so real times sit well above these figures. On a full 8-wide RAIDZ2 pool the rebuild reads 224TB from the seven surviving drives while the pool runs on one parity drive.
Sequential rebuild, zpool replace -s, does not help here: the zpool-replace man page says it is not supported for RAIDZ. It applies to mirrors and dRAID only. That leaves the pool layout as the lever:
- 4 to 8 drives: RAIDZ2 as the minimum at 30TB per disk; RAIDZ1 leaves no margin for a second failure during a day-and-a-half rebuild.
- 9 to 14 drives: RAIDZ3, or several RAIDZ2 vdevs so each resilver touches fewer disks.
- 15 drives and up: dRAID with distributed spares, which resilvers into spare space across every disk; you still replace the failed drive afterwards to free that space.
- Mirrors: the fastest rebuild, a straight copy that can use sequential rebuild, at half the usable capacity.
Our RAIDZ2 width guide has the usable-capacity math, and the ZFS RAID levels guide compares the layouts side by side.
Step 6: Pick a drive for a ZFS pool
The IronWolf Pro 32TB is the HAMR drive Seagate markets for NAS boxes of any size, rated for unlimited bays and 550 TB a year with a 5-year warranty. Its single-drive Amazon listing was out of stock on October 21. The only sensibly priced new stock was an IronWolf Pro 32TB four-pack from Adorama at about $5,500 to $5,700, roughly $1,400 a drive. By November 1 the four-pack had risen to about $6,000, while the single IronWolf Pro 32TB was back in stock new from a third-party seller at about $1,400, now the cheaper way to buy one drive. The Seagate launch list price in January was about $850, so the shortage has added more than half again.
The other choices trade capacity or warranty for availability:
- Renewed Seagate Exos 28TB: CMR, about $900 to $920, roughly $32 to $34 a terabyte, from goHardDrive. Seagate does not say whether this recertified model is HAMR.
- WD Red Pro 24TB: about $1,300 to $1,350 and sold by Amazon itself, the only drive in this group that was.
Check the current price on each before you order, since shortage pricing moved daily through 2026. Our recertified drive guide covers how to test a renewed Exos before it joins a pool, and the NAS hard drive guide prices every capacity up to 26TB.
Skip the drives you cannot buy at retail anyway: the HC680, the HC690, the WD 40TB, the Toshiba M12 and the Mozaic 4+ 44TB. If you need a box for eight of these drives, see our 8-bay NAS guide, and for a DIY pool, our TrueNAS ZFS build guide.
