U.2
Direct Answer
U.2 uses the SFF-8639 connector to bring PCIe x4 NVMe speeds to a 2.5-inch, hot-swappable drive body -- the enterprise and datacenter counterpart to consumer M.2, trading M.2's compact size for better cooling, higher power delivery, and the ability to swap drives without powering down.
Source: SFF-8639, December 2011 (renamed U.2, June 2015)
Specs
| Connector | SFF-8639 |
| Max PCIe Lanes | x4 (same NVMe electrical spec as M.2 M-key) |
| Power Rails | 3.3V, 5V, 12V (vs. M.2's 3.3V only) |
| Hot-Swap | Yes |
The Enterprise Answer to M.2
U.2 carries the same PCIe x4 NVMe electrical connection as an M.2 M-key slot, but through the SFF-8639 connector on a standard 2.5-inch (commonly 15mm-thick) drive body rather than a thin M.2 card -- and it supports multiple protocols over that same connector (NVMe, SAS, and SATA), which M.2 doesn't. The original SFF-8639 spec was released in December 2011 and renamed U.2 in June 2015.
That larger body is the whole point in datacenter and enterprise contexts: more PCB area for capacitors and controller hardware, more surface area for sustained cooling under heavy load, and support for 3.3V, 5V, and 12V power rails versus M.2's 3.3V-only limit. U.2 drives are also hot-swappable, letting a failed drive be replaced without powering down the system -- something M.2, mounted directly to the motherboard, was never designed for.