M.2 supports both PCIe and SATA as the protocol. The SSDs in MacBooks used SATA as a protocol until a certain point. Now they use PCIe as a protocol. But that'

M.2 supports both PCIe and SATA as the protocol. The SSDs in MacBooks used SATA as a protocol until a certain point. Now they use PCIe as a protocol. But that's not what I'm talking about, I'm talking about the physical measures of the slot and the form factor of the card. They always have been proprietary although standards that are just as good existed.