Hey folks, Just wanted to say that I did exactly what @dan2h advised and I managed to replace my SSD in my mid-2012 MacBook Pro 15' Retina with a Samsung 850 EVO mSATA. It works fine. Basically you just order the Samsung SSD, then you buy this adapter and follow the instructions in this video. Don't forget to get the special screwdrivers, P5 and T5; I used this one because it was half price. The key point here is that Apple charges £490 to replace the SSD for my particular model (alternatively the OWC and Jetdrive custom-built SSDs are priced in the same range), but I fixed it myself for ~£150. Thanks to everyone whose online footprints I've followed. Hope this helps someone in the future.