WD Ultrastar DC HC550: 14TB, 16TB and 18TB, Against the HC555 and Exos X18

Ultrastar DC HC550 part numbers: 14TB, 16TB and 18TB

WD's sheet gives each HC550 two identifiers: a long model number such as WUH721818ALE6L4 and a short part number such as 0F38459. A listing may print either, so this grid holds both, by capacity, interface and encryption option. We paired them using the sheet's key, where a final 4 means Base SE with no encryption, 1 means SED and 5 means SED-FIPS.

Two details catch people out. SED-FIPS appears on SAS only, so a SATA HC550 is either Base SE or SED on this sheet. And the 18 TB SAS list carries one more model, WUH721818AL5200. WD's key gives a final 0 as Instant Secure Erase, but the part-number table has no column for it, so that model has no short part number here and stays out of the grid.

Capacity sits in the model number itself: WUH721818 is 18 TB, WUH721816 is 16 TB and WUH721814 is 14 TB. Nothing on the sheet reaches 20 TB, so a search for an Ultrastar DC HC550 20TB is really after another model. WD's HC555 sheet lists 20 TB, and so does the Ultrastar DC HC560.

WD Ultrastar DC HC550 on Amazon.co.uk

This table takes Amazon.co.uk listings whose title names the HC550 or one of the sheet's part numbers, and holds 4 listings right now, sorted by pounds per TB inc VAT. Capacity is a column, so filter by size there. If your capacity has no row, the table holds none at this moment. Match each row's number to the grid above, because a number that is neither in the grid nor WUH721818AL5200 is not described by the figures on this page.

Ultrastar DC HC550 SATA or SAS: what the interface changes

Sustained transfer changes with capacity, not interface: 269 MB/s at 18 TB and 262 MB/s at 14 and 16 TB. Idle power is one wattage per interface across all three sizes, so W/TB falls as capacity rises.

Sector format is the practical split. If your controller or array expects 520 or 528 byte sectors, only the SAS column lists them, so check your own system's documents before you pick a SATA number on price. Operating power is 6.5 W on SATA and 8.8 W on SAS, but the tests differ: SATA is random RW 50/50 at 8KB QD1 and 40 IOPS, SAS is random RW 50/50 at 4KB QD4 and maximum IOPS. Do not treat that gap as the cost of the interface alone.

Rated speed also sets a floor on how long a full write takes. By our arithmetic from the sheet's maximums, no full write can beat about 18.6 hours at 18 TB, 17.0 hours at 16 TB and 14.8 hours at 14 TB.

Ultrastar DC HC550 datasheet: design and ratings

WD's sheet, dated May 2023, describes a nine-disk platform in CMR format with a sixth-generation HelioSeal design. WD calls it the first HDD in the industry to use Energy-Assisted Magnetic Recording (EAMR) and the first with a Triple Stage Actuator. Those firsts are the maker's own wording. What a buyer can act on is the list of ratings, which apply to both interfaces.

• Workload: up to 550 TB a year.
• Reliability: 2.5 million hours MTBF and 0.35% AFR, both projected.
• Limited warranty: 5 years.
• Data buffer: 512 MB, with a portion used for firmware.
• Areal density: 1022 Gbits per square inch at 18 TB and 918 at 14 and 16 TB.
• Physical: 690 g maximum, 26.1 mm z-height.
• Acoustics, typical: 2.0 Bels idle and 3.6 Bels operating.

Treat the reliability pair with care. The same sheet says MTBF and AFR do not predict an individual drive's reliability and are not a warranty. They will not tell you how one disk in your server will behave.

Ultrastar DC HC550 vs HC555

The Ultrastar DC HC555 shares capacities with the HC550, and the model numbers are easy to confuse. Compare WUH721818AL with WUH722018CL: the digits after WUH7 and the letter before L both move, and WD's key gives the HC555 generation code C where the HC550 has A. All HC555 figures here come from its sheet, dated November 2024. This page prices only the HC550, so there is no pound comparison between the two.

Both sheets state the same operating tests, so that row is a fair comparison, and it splits by interface: the HC555 is 0.1 W lower on SATA and 0.4 W higher on SAS. On SATA the HC555 also leads at idle, and on either interface it leads on operating noise and on transfer at 14 and 16 TB. At 18 TB that transfer gap closes. SAS idle per terabyte ties at all three shared sizes, though the raw SAS watts differ by 0.1 W, so the tie is rounding.

So on SATA the HC555's figures lead or match on every power and speed line above. SAS is closer, and the HC550 holds two lines there: lower operating power, and the 520, 528, 4160 and 4224 byte sectors, which the HC555 sheet does not show.

One more difference for anyone searching by short number: we have not found HC555 short part numbers in the HC555 sheet text, so a 0F code cannot be tied to that drive from this document. WD does say a 512e HC555 can be converted to 4Kn with its WDCKIT Drive Utility.

Ultrastar DC HC550 vs Exos X18

This is the same-size choice between makers. Both are helium-sealed CMR drives covering 14, 16 and 18 TB, and both state 2.5 million hours MTBF, 0.35% AFR and a 5-year limited warranty, so those headline ratings settle nothing. The real differences are below, each side taken from its own maker's sheet.

Seagate's sheet shows lower idle power at every point in this grid, and the gap widens as capacity falls because the HC550 holds one figure across its range. Transfer is within 1 MB/s at 18 TB, with an 8 MB/s lead for the X18 at 14 and 16 TB. The HC550 answers with double the memory on paper, though the two sheets label it differently and WD reserves a portion for firmware. We have not found a document measuring what that difference does under a real workload.

Why idle only against the X18: WD's SATA operating figure uses 8KB QD1 at 40 IOPS, whilst Seagate quotes maximum operating power at 4K/16Q. Those are different tests, so subtracting one from the other would say nothing useful.

Sector options line up closely. On SAS, both sheets add 520 and 528 at 512e and 4160 and 4224 at 4Kn, and both keep SED-FIPS away from SATA. Seagate's FastFormat models ship in 512e, and switching to 4Kn deletes all data on the drive, so make that change before the disk holds anything. WD rates the HC550 up to 550 TB a year; we have not found a workload figure in the X18 sheet text we hold, so that row stays out of the comparison. Seagate's side has its own Exos X18 page, with Amazon.co.uk rows for that family.

New HC550 listings, and what the warranty line covers

This narrower table keeps only new-condition listings and holds 3 listings right now. WD's sheet states a 5-year limited warranty, but we have not found a WD document setting out UK terms for the HC550, or one saying whether that warranty reaches a used row. We have not found a WD document on recertified HC550 drives either, so a used row in the wider table is simply used, and it proves nothing about factory work.

WD's sheet states CMR for this family, while a CMR label in an Amazon title is a claim by whoever wrote that title. The Recording column prints a technology only where a document ties it to the part number, so a row without one marks our sourcing gap and is not a finding about the drive.