· Atomic Racks
WD Ultrastar DC HC530 vs HC550 vs HC570
The WD Ultrastar DC HC530, HC550 and HC570 are three generations of the same 7200 rpm helium enterprise platform, at 14 TB, 16 to 18 TB and 22 TB respectively. Used, the HC530 and HC550 are the value picks and are interchangeable in an array; the HC570 is for builds that need maximum capacity per bay.
Three generations, one platform
Western Digital's Ultrastar DC HC500 series is the capacity-optimized enterprise line, the direct counterpart to Seagate's Exos X series. Each generation is sealed with helium, spins at 7200 rpm, uses conventional magnetic recording (CMR), carries a 550 TB per year workload rating, a published MTBF of around 2.5 million hours, and a five-year original warranty. All three ship in SATA 6 Gb/s and SAS 12 Gb/s versions and in 512e and 4Kn sector formats.
What changes between generations is how WD reached the next capacity step. The HC530 reached 14 TB with eight platters. The HC550 moved to nine platters and energy-assisted recording for 16 and 18 TB. The HC570 reached 22 TB on ten platters with WD's OptiNAND architecture, which adds embedded flash for metadata.
Spec comparison
| Specification | DC HC530 | DC HC550 | DC HC570 |
|---|---|---|---|
| Capacities | 14 TB | 16, 18 TB | 22 TB |
| Spindle speed | 7200 rpm | 7200 rpm | 7200 rpm |
| Cache | 512 MB | 512 MB | 512 MB |
| Recording | CMR | CMR with energy-assisted recording | CMR, energy-assisted, OptiNAND |
| Fill | Helium | Helium | Helium |
| Interfaces | SATA 6 Gb/s, SAS 12 Gb/s | SATA 6 Gb/s, SAS 12 Gb/s | SATA 6 Gb/s, SAS 12 Gb/s |
| Max sustained transfer | Around 265 MB/s | Around 270 MB/s | Around 290 MB/s |
| Workload rating | 550 TB/year | 550 TB/year | 550 TB/year |
| Original warranty | 5 years | 5 years | 5 years |
Transfer figures are datasheet maximums for the top capacity in each family. Real sustained throughput on a used drive should land within roughly 10 percent of these on the outer tracks and taper toward the inner ones.
Model numbers on the used market
WD part numbers run long but decode cleanly. WUH72 is the Ultrastar helium 7200 rpm prefix, followed by capacity, then interface and sector format codes.
- WUH721414AL5204: HC530 14 TB, SAS 12 Gb/s, 512e. One of the most common SAS datacenter pulls.
- WUH721414ALE6L4: HC530 14 TB, SATA, 512e.
- WUH721818AL4200: HC550 18 TB, SAS 12 Gb/s, 4Kn.
- WUH721818AL5204: HC550 18 TB, SAS 12 Gb/s, 512e.
- WUH721818ALE6L4: HC550 18 TB, SATA, 512e.
- WUH722222ALE6L4: HC570 22 TB, SATA, 512e.
In the suffix, AL means SAS and ALE means SATA. The following digits encode sector size and features: 5204 is 512e SAS, 4200 is 4Kn SAS, and E6L4 is 512e SATA. If a listing shows a suffix you do not recognize, check it against the Western Digital product page at westerndigital.com before assuming it will present 512 byte sectors to your controller.
512e versus 4Kn
This matters more for Ultrastar drives than for Exos because 4Kn SAS versions are common in datacenter pulls. A 4Kn drive presents 4,096 byte logical sectors. Linux, FreeBSD, TrueNAS and current Windows handle 4Kn drives on a modern HBA without trouble. Older RAID controllers, some USB enclosures and legacy boot firmware do not. If your controller is more than a decade old, buy 512e. If you run a current HBA in IT mode, either works, and 4Kn units are sometimes cheaper.
Used SAS drives of any generation can also arrive formatted to 520 or 528 byte sectors from storage arrays. Those need reformatting with sg_format before any operating system will use them. See using a SAS drive in a desktop or NAS.
What the recording technology changes
The HC550 introduced energy-assisted magnetic recording (WD's term for its first-generation assisted technology) to write narrower tracks reliably. The HC570 adds OptiNAND, which stores track-level metadata in onboard flash rather than on the platters, freeing surface area and letting the drive manage adjacent-track refreshes more efficiently. Both are transparent to the host: the drive still behaves as a CMR device with no shingled write penalty.
For a used buyer the practical implications are small. The newer technologies have shorter field histories, so there is less observational data on long-term behavior. Nothing published suggests a reliability problem with either, and the HC550 in particular has been shipping in volume for years.
Which to buy used
- HC530 (14 TB). The budget pick per terabyte. Mature, widely available in SAS, usually with 25,000 to 45,000 hours. Ideal for RAIDZ2 pools where bay count is not the constraint.
- HC550 (16 or 18 TB). The sweet spot. 18 TB per bay with a mature platform, often available in both Light Use and Standard tiers. The 4Kn SAS version is frequently the cheapest route to 18 TB.
- HC570 (22 TB). Buy when bays are scarce or when you are consolidating many smaller drives. Newer, fewer used units, higher cost per terabyte. Resilvers take proportionally longer.
Across all three, the condition checks are the same as for any enterprise drive: zero grown defects or reallocated sectors, a completed extended self-test, helium level at 100, and a benchmark near the datasheet figure. Browse current WD Ultrastar stock across all three families.
FAQ
Can I mix HC530 and HC550 drives in one pool?
In separate vdevs, yes. In the same vdev, only if the capacities match, which means 14 TB HC530 with 14 TB from another family, since the HC550 was not sold at 14 TB.
Is the 512 MB cache an advantage over Exos drives?
Not in a measurable way for NAS use. The larger buffer helps with write coalescing under some workloads, but sustained throughput is limited by the platters, not the cache.
Are HC550 drives with EAMR less reliable?
There is no published evidence of that. The technology has been in volume production for several years and WD rates the HC550 identically to the HC530.
Why are 4Kn SAS Ultrastars cheaper?
Fewer buyers can use them. 4Kn needs a reasonably modern controller and OS. If you have those, the discount is free money.
What we do at Atomic Racks
Every used drive we sell gets a SMART extended self-test, a fio benchmark and NIST SP 800-88 sanitization with read-back verification, and 520 or 528 byte SAS drives are reformatted before listing. Hours are read from the drive and never reset, and each unit is priced into a power-on-hour tier. The QR tamper seal on each certified unit opens its public test report. 90-day warranty on standard items, up to 2 years on certified tiers, 30-day returns and free shipping to the lower 48. Details on how we test.