Does a M.2 2280 SSD fit in a MacBook Pro (Retina, 15-inch, Mid 2014)? I have a standard 256GB SSD right now. I would like to upgrade that to a 512GB or higher. When I was looking around I found this SAMSUNG 950 PRO 512 GB SSD that has a huge performance in write and read speed.

This SAMSUNG 950 PRO 512 GB has a M.2 2280 connection.
Is it possible to fit this inside my Macbook?
The Samsung 950pro has the same connector and the same chipset.  In fact, both the 950pro and the Samsung manufactured Apple SSD of this type share the same exact PCB and chips.  Initially the 950pro was manufactured as an OEM drive especially for Apple.

However the 950pro retail version has minor hardware differences and a complete different firmware.

While they indeed fit physically, they are NOT compatible.  There are currently no aftermarket drives for this range.  If someone would release a compatible drive in the future, the OWC Envoy range would be my best bet.