I have a late 2015 27" iMac and bought a Samsung EVO 500gb SSD to replace the original 1TB drive. I also got an enclosure. I was able to erase the SSD drive and install Sierra without any issues and use the drive just fine from the enclosure via USB 3.0. I then decided to open up the iMac and install the SSD with the OWC cable. All went fine, but when I try to boot it up, it will show the no entry sign and flash the Apple logo as though it's trying to boot up, but will not actually manage it. I'm currently having to use the old HDD in the enclosure until I find out why. I have not stuck the screen in place yet. HELP!