Running on a macbook pro 13" mid 2009, using El Capitan 10.11.6, 2.53 Ghz intel core 2 duo, 8Gb 1067 MHz DDR3. So I have a rather interesting case where my laptop froze last week, and when i forced shutdown and restarted my computer stopped seeing my hard drive. When it booted up the screen showed a folder with a question mark on it. I tried booting into single user, or safe mode, verbose mode. nothing worked. I cleared nvram and the smc power thing. Basically I tried all the standard trouble shoot command and some other key code. Nothing worked. Interestingly I plugged a back up drive that had a time machine backup with a backup from Snow Leopard. holding option I was able to boot from my external drive, using the time machine backup (which i was under the impression wasn't possible, so already thats odd). Going into disk utility in this snow leopard external drive machine, I saw the laptop wasn't reading the internal drive (AKA not showing in disk utility). I shut down. Swapped ram to see if somehow that caused the issue. Nothing. Things get odd at this point. I borrowed a friends mid 2010 macbook pro 13". I took out his hard drive, popped it into mine macbook and the macbook started just fine. Great, so i figured it must be my drive. I plug my drive into his laptop, and it starts fine. What? Why? I restarted his macbook with my drive in it several times to see if it would fail. It loaded quickly and efficiently as I would normally expect on my macbook. While my drive was still in his macbook I ran disk utility and used first aid on the drive. Everything came back normal. First aid said the drive was fine. So i put my drive back in my laptop, and put his back, and again my macbook still won't read my drive, and I still see the flashing folder. I bought an external drive enclosure, and right now I can use my drive using the external case. Ive runs disk utility a hundred times now, and see no issues. TO back up a bit, I did manage to get verbose mode running a few times, and I did manage to get the laptop to the login screen a few times with just my hard drive inside, nothing connected. Those were random, but the laptop would shutdown everytime I got to login. TO do that I had to reset the firmware - command option f o. for some reason that would get me to at least the apple loading screen, and it would shut down. It didn't always work though. In verbose mode I saw a lot of errors. it couldn't load PlatformSupport.plist, said must be out of unencrypted bootable drive. There was an error loading relinked kernel cache. Recovery image verification fail warning also. There is a ton more. I have a lot of pictures of code that I can't read to diagnose from verbose mode from these instances of getting the drive to read randomly. I can post them if people want. The one time single user mode ran, I couldn't mount the internal disk. It didn't see it. (Single user mode only ran when I had the drive with the time machine snow leopard back up connected.) Im assuming a hardware issue, but it doesn't seem to be with my drive. IM guessing its the SATA cable, except why would another drive work just fine everytime I plug it in? Possible a logic board failure, but again, why would my friends drive run every time no problem? Any suggestions .