M.2 vs U.2 and other SSD form factors
M.2 dominates consumer builds — but the keying, length, and whether the slot runs NVMe or SATA matters before you order.
1. M.2: the consumer standard
M.2 is the form factor for nearly all consumer NVMe drives and many SATA SSDs. The standard size is 2280 (22 mm wide, 80 mm long). Some compact builds use 2242 (42 mm) or 2230 (30 mm) — common in laptops and mini-PCs. The key notch determines compatibility: M key accepts NVMe (x4) and some SATA; B+M key accepts SATA and NVMe x2. Most desktop motherboards use M key for NVMe.
2. Multiple M.2 slots: bandwidth sharing
High-end motherboards offer 4–5 M.2 slots but not all run at full PCIe 4.0 x4 simultaneously. Slots fed by the chipset (not directly from the CPU) share bandwidth through a DMI link. On consumer platforms, only the slot closest to the CPU typically delivers full-speed PCIe 5.0 or 4.0. Check your board's block diagram before buying multiple fast NVMe drives.
3. U.2 (SFF-8639): enterprise workhorse
U.2 is a 2.5" enterprise form factor with a SFF-8639 connector. It delivers PCIe x4 NVMe or SAS bandwidth through a cabled connection, making hot-swap possible. U.2 drives are larger, have better thermals, and typically offer much higher endurance than M.2. They require a U.2 host adapter or a motherboard with native U.2 ports. Rare in consumer builds; common in workstations and servers.
4. Add-in-card (AIC) SSDs
AIC SSDs install in a PCIe x4 or x16 slot (using x4 lanes). They are useful when M.2 slots are occupied, when you need a larger thermal footprint, or in servers with many PCIe slots but no M.2. Some high-end consumer drives ship in AIC format for enthusiast builders. Performance is identical to an M.2 drive on the same interface generation — the form factor is just different.
5. E1.S and EDSFF: data center only
EDSFF (Enterprise and Data Center SSD Form Factor) standards like E1.S are designed for dense data center deployments with standardised airflow and power budgets. Not relevant for consumer or workstation builds. Mentioned here for completeness — if you see them in a spec sheet, they are targeting server infrastructure.
Bottom line
For consumer builds: M.2 2280, verify M-key NVMe support on your board. For workstations with high endurance or hot-swap needs: U.2. For builds with full M.2 slots: AIC. Everything else (E1.S, E3.S) is enterprise/data-center territory. Always cross-reference your board's M.2 slot specifications before purchasing.