U.2 and U.3 drive form factors
U.2 and U.3 are 2.5-inch enterprise SSD form factors that carry NVMe over PCIe. U.3 uses the newer SFF-TA-1001 connector so one tri-mode backplane bay accepts NVMe, SAS, or SATA; U.2 bays are wired for NVMe or SATA only.
Both look like a thick 2.5-inch drive and support hot-swap, power-loss protection, and enterprise endurance. The difference is the backplane. A U.2 backplane routes fixed lanes per bay, so a bay is either NVMe or SAS/SATA depending on how it was built. U.3 unifies the pinout, letting a tri-mode HBA or RAID controller auto-detect whatever drive you plug into any bay.
U.3 drives are generally backward compatible into U.2 backplanes, but a U.2 drive does not gain U.3 flexibility. The flexibility lives in the backplane and controller, not the drive alone. That distinction trips up a lot of buyers who assume U.3 drives create U.3 behavior on their own.
When configuring a server from China, ask specifically whether the backplane is tri-mode U.3 and which controller drives it. Mixing drive types in one chassis is where U.3 pays off. Confirm the model, because vendors sometimes list U.3 drives sitting in a plain U.2 backplane.
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