Question: what is the difference between m.2 and m.2 b+m?
Answer: A 'B' keying (pins 12-19) gives PCI Express SSDs up to 2x lanes of bandwidth, while a 'M' keying (pins 59-66) provides PCI Express SSDs with up to 4x lanes of bandwidth. Even with 2x lanes of bandwidth a 'B' keyed M. 2 SSD still gives 10Gbit/s performance, whereas the 4x lanes on 'M' gives up to 20Gbit/s.