Accordingly, M.2 is a small-sized disk that directly connected to the motherboard. While U.2 can be defined as a high capacity disk connected to workstations and server motherboards. U.2 SSD is much faster than M.2 SSD. U.2 used 4 PCle lanes, giving 4GB/s for data read and write while the M.2 SSD only has around 2500MB/s speed.