Diffferece between U.2 and SATA E interface

The physical interface of SATA Express is actually a modification of the SATA I interface. It uses both the SATA I interface and a mini SATA interface with only 4-pin connectors. The mini interface can only accommodate PCI-E lines. The benefit of this approach is to maintain backward compatibility because currently, there are very few SATA E hard drives available, or it can be said that there are hardly any officially commercialized models. By doing this, even if users don’t have SATA Express interface hard drives, SATA E can still be used as two SATA I interfaces, preventing any wastage.

The U.2 interface shares a similar concept with the SATA E interface, both aiming to make the most of existing physical interfaces. However, to achieve faster bandwidth, the U.2 interface has evolved from PCI-E x2 to PCI-E 3.0 x4. Additionally, it has added support for various new protocols, such as NVMe, which SATA E lacks. Hence, U.2 can be considered as the ultimate evolution of SATA E.

The U.2 interface on the device side combines the characteristics of both SATA and SAS interfaces, filling the gaps with pins left by the SATA interface. It also incorporates an L-shaped key design to prevent incorrect connections, allowing compatibility with SATA, SAS, and SATA E specifications. On the motherboard side, it uses the miniSAS (SFF-8643) interface, while the U.2 cable on the device side connects to the SATA power and the data port of the U.2 hard drive.