Was trying to install a new Samsung EVO 850 SSHD and the Mac would not read or recognize the new drive. After reading some forums I found that the 2012 MacBook Pros had issues with the SATA cable.
The repair was easy. Just remove a few screws peel the old SATA cable out. Replace it with the new one paying attention to the IR sensor and making sure it's plugged in correctly.
You can use a pencil or fine tipped Sharpie to trace the old cable before you remove it to make sure the new one lines up perfectly.