Samsung SSD 840 EVO not detected in MacBook Pro 2012.

Hello,

In January I had a problem with my computer where it would overheat, the battery would die quickly, and the overall computer speed was slower than a turtle for even the most basic functions. I brought my computer to a MacMan retailer and they fixed the issue. I then got my MBP back and it worked fine for about a month. It then repeated the same symptoms it had before I got it fixed. I talked to my roommate and did some research and ended up purchasing the Samsung SSD 840 EVO 250gb to replace my Hard Drive.

I transferred all my data to the new SSD and then installed the SSD to the internal bay where the hard drive sits without any problems. After the SSD was in place I booted up the computer and I got a "flashing folder with question mark." The SSD was not internally recognized by my computer. I then tried to put my Hard Drive back in to see if it would be recognized and it was not. However, both the SSD and Hard Drive are externally recognized via USB enclosure. I have been working on this for a couple days and am getting frustrated.

I have tried a number of fixes I thought would surely work. I reformatted the SSD to the appropriate Mac format that every forum and website recommend: Mac OS Extended (Journaled) and that does not work. I have reinstalled OS X Yosemite, I have tried updating the SSD Firmware through USB. Nothing has worked yet. I have tried using Apple's internet recovery system and neither drive is recognized when inside the computer. My best assumption but least favorable is the Cable within the MacBook Pro that connects to the Hard Drive/SSD is faulty. I fear that it may be faulty since now neither the SSD or Hard Drive is detected when they are installed in the MacBook Pro, only when external.

Can anyone please help me figure out what could be wrong? If you need more information just ask and I will do my best to answer them to your satisfaction. Anyone input, conclusion, or direction would be greatly appreciated!

Thank You for your time and knowledge!