I've got a mid-2012 unibody macbook pro 13". I replaced the original HDD with a samsung SSD and it worked perfectly running El Capitan for 3 months. However- now it doesn't boot and comes up with a gray screen with the prohibition or 'cancel/stop' sign. I have my old HDD which boots externally in an enclosure, but doesn't boot when I put it back in the laptop. The SSD doesn't boot internally or in the enclosure. I cannot repair the SSD with disk utility, as it cannot be fully verified. I tried to erase the SSD (which worked) but a clean install of El Capitan from a bootable USB drive failed and now the SSD boots with a folder and flashing 'question mark'. Any ideas what to do next? I would naturally assume it's the SATA cable- but the SSD is recognised internally (desktop icon) when I boot and log on from the HDD in the enclosure!