Hi! I'm a Computer Engineer, Nice to answer all doubts and questions about Mac's, put an SSD to MacBook Pro isn't as easy as it looks, the problem described above could be a Software problem or just simply a SMC reset or NVRAM restart. I have been reading all of the answer, and unfortunately, i have to correct a very big concept mistake of the answer posted on (08/31/2015) . "the cables specs were for SATA II not SATA III drives" ==> That is not true, it's complete false. FIRST: In Serial - ATA interfaces there aren't different type of cables according of the bus speed. The bus can be SATA I (up to 1.5Gb/s) , SATA II (up to 3 Gb/s) or as far, SATA III (6 Gb/s). This speed it depends of the chipset of the computer, in this case, we are talking about MacBook Pro Mid 2012 with have Intel 7 series chipset, which full support SATA III. SECOND: Even deferents types of bus and also, speeds, the CABLE IT'S THE SAME, there are not difference between SATA gen, there used to be deferents cables in P-ATA interfaces...