U.2 SSD Buying Guide: Connector, Dimensions, M.2 Comparison and UK Prices

U.2 SSD connector: SFF-8639 by its older name

A U.2 SSD plugs in through the connector maker sheets call SFF-8639. A 2018 SNIA deck gives the naming directly: U.2, formerly SFF-8639, and always pronounced U dot 2. Samsung's form-factor table lists SFF-8639 as the U.2 connector, and Micron's part-number chart describes the 9400 as U.3/U.2, 2.5 inch, 15 mm, SFF-8639. We have not read the SFF-8639 specification itself, so this page prints no pin layout and makes no claim about cables.

The shape has a history that explains its limits. SNIA gave U.2's design priority as compatibility with the ubiquitous 2.5-inch hard drive, and a 2022 SNIA and Flash Memory Summit slide says 2.5-inch devices evolved into PCI Express U.2. The KIOXIA, Meta and Microsoft white paper is blunter: the 2.5-inch form factor came from hard drives, is not optimal for flash and cannot scale beyond four PCIe lanes. Samsung's table also lists four lanes for U.2. For a single drive, that is simply the link you are buying.

U.2 SSD dimensions and form factor: 7 mm and 15 mm

U.2 does not fix one thickness. These two sheets describe the same 2.5-inch family at two heights:

Kioxia's CM7-R, which its brief labels U.3, sits in the same 15 mm envelope: 15.0 mm thick, 69.85 mm wide, 100.45 mm long and 130 g at most. Samsung's white paper gives U.2 width and length as 69.85 x 100.2 mm. So length and width barely move across makers while thickness more than doubles (our arithmetic). Thickness is the number to carry to your host's manual. Lenovo's SR650 V3 guide, for one server, lists two 7mm drives at the rear of the server, in addition to any 2.5-inch or 3.5-inch drive bays, and says it supports up to two externally accessible 7mm hot-swap drives for operating system boot functions. We make no claim here about 15 mm bays.

Capacity ceilings are a second trap, with three figures from one maker. Samsung's PM9A3 data sheet lists U.2 from 960GB to 7.68TB. Its white paper says capacities run to 15.36TB for U.2. Its form-factor table gives a 32TB ceiling for U.2, which describes the form factor rather than a model. We print all three rather than pick one. Elsewhere, Micron's 9400 PRO lists 7,680 GB, 15,360 GB and 30,720 GB.

U.2 SSD vs M.2: what Samsung's table separates

The upper rows come from Samsung's form-factor table. The last two are one family, the PM9A3, on Samsung's own figures. Both shapes list four lanes in that table, so the gap in that family is not a matter of lane count. M.2 also comes at other lengths: Micron's 7450 sheet lists both 22.00 x 80.00 mm and 22.00 x 110.00 mm.

The rest of the difference is where the drive lives. The KIOXIA, Meta and Microsoft white paper says M.2 needs a connector slot on the motherboard, that its 22 mm width constrains flash placement and limits high capacities, and that replacing one normally means powering the whole server down. A U.2 buyer is choosing a bay, and an M.2 buyer a slot on the board.

One caution on hot-swap. Micron's 7450 sheet lists surprise insertion and removal and hot-plug capability for a series sold in M.2 and E1.S, which reads differently from Samsung's M.2 mark. Treat them as two makers' statements and go by the sheet for the part in front of you.

U.2 SSD speed: the PCIe generation, not the shape

U.2 is a shape and a connector. Speed comes from the drive and its PCIe generation. On PCIe Gen4, Samsung's data sheet gives the PM9A3 6,500 to 6,900 MB/s sequential read across its four sizes, Micron gives the 9400 up to 7,000 MB/s and Kioxia gives its U.2 CD8 up to 7,200 MB/s. On PCIe 5.0, Kioxia's CD8P, offered in U.2 and E3.S, reaches up to 12,000 MB/s. These are makers' figures and not our tests.

You may also meet 14,000 MB/s for Kioxia's 2.5-inch CM7-R on PCIe 5.0. Its brief labels that drive U.3, so we do not count it as a U.2 figure. Only Samsung's and Micron's data sheets above give full sets of figures for this page's Gen4 examples, and we have read no U.2 sheet from Kioxia beyond the brief's summary box.

Hot-plug and power loss protection on U.2 drives

Samsung's table marks U.2 as hot-swap, and Micron's 9400 sheet lists surprise insertion and removal and hot-plug capability. Those statements are about the drive. Whether your backplane and operating system allow a live removal is a question for the host's documents, and we hold none that settle it in general.

The KIOXIA, Meta and Microsoft white paper describes power loss protection as using the backup power of a solid capacitor to record data held in buffer memory to NAND flash after a sudden shutdown. Samsung says the PM9A3 includes enterprise PLP, and Micron states full power-loss protection for the 9400. Both are family statements, so confirm them for the part number. Endurance grades are covered on enterprise NVMe SSDs.

U.2 SSD hosts: servers, U.3 bays and NAS

Do not assume a bay that looks right will take a U.2 drive. Lenovo's SR650 V3 guide says that in its tri-mode configuration only NVMe drives with a U.3 interface are supported, and U.2 drives are not. That exclusion belongs to the tri-mode configuration: as an alternative, the guide says the RAID 9450-32i adapter supports connectivity to either NVMe U.2 and U.3 drives. Quarch said in April 2019 that U.2 drives are not compatible with U.3 hosts, while U.3 drives remain backward compatible with U.2. We have not read the specification behind that, so your host's own document decides. The other direction is covered on U.3 SSDs.

For a U.2 SSD in a NAS we hold one example. QNAP lists its TS-h1290FX with twelve 2.5-inch bays for U.2 PCIe NVMe or SATA 6Gbps drives, says the system ships without SSDs and points to its compatibility list. That does not make any row below compatible with that NAS. Take the exact part number to QNAP's list, and treat any other NAS as a question for its own maker.

U.2 SSD price: live listings on Amazon.co.uk

The table holds 9 listings right now. Lowest per TB: £187.00/TB. Prices are pounds sterling inc VAT, as Amazon.co.uk lists them at the time shown at the top of the page. The table takes listings recorded as NVMe with a U.2 form factor. A 2.5-inch NVMe listing whose record does not state U.2 stays out, even if the maker's sheet might call that drive U.2, so a drive missing here may still be on sale.

The table has no thickness column, so take that from the maker's sheet for the part number. A listing recorded as U.3 is on the U.3 page rather than here.

How to choose a U.2 SSD

• Host: confirm in its documents that the bay takes U.2 and is not a U.3-only tri-mode setup.
• Thickness: 7 mm and 15 mm both exist, so read the sheet and then the host's manual.
• PCIe generation: Gen4 and Gen5 U.2 drives both appear on the sheets, and the host's link matters as much as the drive's.
• Capacity: read it from the part number, not from a form-factor ceiling.
• Power loss protection and hot-plug: confirm both on the maker's sheet.
• NAS: go by the NAS maker's own list, as QNAP's page directs.
• Condition: read that column, and the seller's wording on Amazon.co.uk, before you press Buy on Amazon.