You can't put something as tall as a M.2 connector on the back side of an expansion slot without violating the form factor guidelines and enchroaching on the space of the next slot over. The only compliant way to put M.2 drives on the back is to use an offset edge connector so the main board is a bit lower than it usually would be. Amfeltec has some boards like this, but I think they have a patent on their offset connector. http://amfeltec.com/products/pci-express-gen-3-carrier-board...