I was trying to replace my original HDD with a SSD (Samsung Evo 860 500Gb). I could boot up fine on the new SSD via USB (using a USB-SATA cable) but every time I tried to boot when the SSD was inside my computer (in the place of the original HDD), I couldn’t boot (forbidden sign appeared). I read on several forums it was a compatibility problem with the original SATA cable. I seemed strange to me but I decided to buy a new SATA cable via iFixit. I’ve just replaced the original SATA cable with the new one and now I can boot on the SSD without any problem !
Thank you very much for this tutorial, even if I still don’t understand why it doesn’t work with the original SATA cable ^^