So I was just using my macbook pro like I usually do. Watching Netflix then all of a sudden the page crashes and the screen is just white with a circle and a diagonal line through it. (Sort of like when you are driving, its like the sign that basically means do not enter.) So I held down the power button and to turn off my macbook and turn it on again. Everything loaded up normal and I was at the login screen. Put in my password and it started to load up, slower though, then it got about half way through the bar and the sign was up again. Thought my hard drive had died. This was an SSD that I used to replace the stock HDD two years ago. So I put that old HDD back in and it loaded up OS X fine. Put my SSD in an external case and plugged it in to see if maybe I could recover my files. To my surprise it loaded up my entire file system perfectly so I took all my important files and transferred them over. Just to see if it would work I shut off my macbook pro and turned left the ssd plugged and and held option on start up to select the SSD. It loaded up again and I typed the password and everything was working normally. Since it was working now, I thought it might have been a fluke thing that the no enter sign kept popping up. So I put the SSD back inside the macbook pro and loaded it again. But this time the no enter sign popped up again. I decided this time to give it some time before I shut it off. After about 5 minutes it loaded everything up again but it was painfully slow. Got to the point where it actually froze so I turned it on/off again. Did this about 10 times and all 10 times it showed the no enter sign. I waited for 5 minutes and only 4 times OS X loaded up but it was really slow the others time it just stayed on that no enter sign. So I took it off and put it in the external casing again. Loaded it from there and first try it loaded up normally and everything was working great. Rebooted a couple times to make sure. Turned if off and on. No matter what I did, it would still work great while I booted it from external. Has this ever happened to anyone or does anyone know why this might happen? I have been searching every where and can't find an answer. My macbook pro is a mid-2012 i7 2.9ghz model, the one right before the retina came out. I was on the latest version of El Capitan. This macbook has been working great ever since I bought it and out of no where the no enter sign appear. The only thing I had open was safari on netflix. Any help would be appreciated.