E3.S vs U.2 vs U.3: Which SSD Form Factor for a New Server?

Form factor is decided by the server, not the drive. Once you choose a chassis and backplane, you are buying that drive shape for the life of the machine, so it deserves attention at server specification time. The short version: U.2 and U.3 are both 2.5-inch NVMe drives on the same connector, and U.3 is the more flexible choice for new drive purchases. E3.S is the EDSFF form factor that current PCIe 5.0 servers from Dell and HPE offer as a front-bay option, and it is the better choice for a new server built around dense NVMe. Here is how they differ and how to decide.

What is the difference between U.2 and U.3 SSDs?

Physically, very little. Both are 2.5-inch drives that use the SFF-8639 connector and normally run NVMe over a PCIe x4 link. The difference is how the connector's pins are assigned and what kind of backplane the drive expects.

• U.2: the original 2.5-inch NVMe standard. A U.2 bay carries dedicated PCIe lanes for NVMe on their own pins, separate from the pins SAS and SATA drives use, so NVMe and SAS/SATA usually need separate backplane wiring and controllers. Examples include the Samsung PM9A3 960GB and Samsung PM1733 1.92TB, both PCIe 4.0 x4.
• U.3: defined by the SFF-TA-1001 specification, it routes NVMe over the same pins used by SAS and SATA. Paired with a tri-mode controller, one U.3 bay accepts any of the three drive types. Examples include the Micron 7450 PRO 960GB, Micron 9400 PRO 7.68TB and the PCIe 5.0 Samsung PM1743 1.92TB.

Are U.3 drives backward compatible with U.2 bays?

In most cases, yes. U.3 drives are designed to detect the host and work in U.2 NVMe bays as well as U.3 tri-mode bays, which is one reason many current NVMe drives ship as U.3. The reverse does not hold: a U.2 drive generally will not work in a U.3 tri-mode bay, because it expects PCIe on pins that the tri-mode backplane does not use for NVMe. Two caveats apply. First, confirm against your server vendor's drive matrix, because OEM platforms can restrict drives by qualification as well as by standard. Second, "U.2" is sometimes used loosely for any 2.5-inch NVMe drive, U.3 models included, so check the form factor field in the drive's specifications rather than the listing title. Our product pages show it in the specifications, for example U.2 SFF on the Samsung PM9A3 and U.3 SFF on the Micron 7450 PRO, and the 2.5-inch enterprise SSD listing lets you compare them side by side.

What is E3.S (EDSFF), and why are new servers adopting it?

EDSFF (Enterprise and Data Center Standard Form Factor) is a family of drive shapes designed for flash from the start rather than inherited from hard drives. It includes E1.S and E1.L, long, thin drives aimed at dense 1U designs, and E3.S and E3.L, aimed at mainstream 1U and 2U servers. E3.S comes in 1T and 2T thicknesses, supports x4 and wider PCIe links, and is also used for CXL memory expansion devices.

The benefits are thermal and electrical. EDSFF drives are designed around front-to-back airflow and the power levels of high-performance NVMe, which matters as PCIe 5.0 drives run faster and hotter. The edge connector is built for PCIe 5.0 signaling and beyond, and the bays are NVMe-only, so there is no tri-mode compromise to design around.

Which servers offer E3.S bays today?

• Dell PowerEdge R760: up to 16 EDSFF E3.S Gen5 drives in front (122.88TB listed), as an alternative to 24 2.5-inch bays (368.64TB listed), plus up to 4 rear E3.S or 2.5-inch drives.
• Dell PowerEdge R760xa: up to 6 E3.S Gen5 NVMe (46.08TB listed), 6 2.5-inch NVMe (92.16TB) or 8 2.5-inch SAS/SATA/NVMe drives in a GPU-optimized 2U chassis.
• HPE ProLiant DL360: up to 20 EDSFF NVMe drives in 1U, compared with up to 8 SFF (plus 2 optional) NVMe/SAS/SATA bays.
• HPE ProLiant DL380 and HPE ProLiant DL385: up to 36 EDSFF drives in 2U, compared with 24 front SFF bays.

Look closely at Dell's capacity figures. The E3.S maximums work out to 7.68TB per drive and the 2.5-inch maximums to 15.36TB per drive, which reflects the largest drives qualified for each bay type in those listings rather than a limit of the form factor. If your design depends on very high capacity per drive today, confirm which drive sizes are qualified for each bay option before you choose.

Should my next server use E3.S (EDSFF) drives?

• Choose E3.S if you are buying a new PCIe 5.0 server for NVMe-heavy work (databases, virtualization, AI data staging), want the most NVMe drives per rack unit, and are willing to standardize on E3.S spares for the server's life.
• Choose U.3 if you want one bay type that accepts SAS, SATA and NVMe, need to reuse existing 2.5-inch drives, want the widest choice of drive models, or need the highest capacity per drive your vendor currently qualifies.
• Stay with U.2 only when adding drives to an existing U.2 NVMe server. For new drive purchases into U.2 bays, a U.3 drive is usually the more flexible buy, subject to the server's qualification list.

Pick the form factor when you pick the chassis: E3.S for new NVMe-first servers, U.3 for flexibility and mixed estates, U.2 only to extend what you already own.

Where do E1.S, M.2 and add-in cards fit?

E1.S appears in hyperscale and GPU platforms where many small NVMe drives fit a 1U sled. Each compute tray in the NVIDIA DGX GB200 NVL72, for example, carries four 3.84TB E1.S NVMe drives for local storage plus a 1.92TB M.2 NVMe boot drive. M.2 remains the standard for internal boot devices, and HHHL add-in cards add NVMe to servers with no NVMe bays at all, at the cost of hot-swap. None of these are interchangeable with 2.5-inch or E3.S bays, so treat them as separate spare pools. For how Gen5 drives change storage design more broadly, see our guide to PCIe Gen5 NVMe performance.

What should you lock down with the server order?

• Bay type and count for each zone (front, rear, mid-chassis), and whether each zone is SAS/SATA, NVMe, tri-mode or EDSFF.
• The controller path: direct CPU attach, PCIe switch, or tri-mode RAID controller or HBA. It decides which drives work and at what PCIe generation.
• Drive carriers. OEM servers need the vendor's tray or carrier, so confirm whether drives ship with the one your platform needs.
• Spares strategy. Hold spares in the same form factor and, ideally, the same model and firmware as the installed drives.
• The qualified drive list, including maximum capacity per bay type.
• Growth plan. If you expect to add drives over the server's life, order enough bays of the right type now; changing a backplane later is a vendor upgrade at best and impossible at worst.

Nexus Compute supplies EDSFF-capable servers from Dell and HPE and 2.5-inch U.2 and U.3 NVMe drives, all new with the full manufacturer warranty. Send the server model and bay configuration through request a quote, including any E3.S drives you need, and we will confirm drive-to-backplane compatibility and return pricing within 48 business hours.