"First I would double check your systems firmware. Follow this Apple T/N: About EFI and SMC firmware updates for Intel-based Mac computers.
I would also replace the SATA cable as well as they do tend to fail.
Update (08/31/2015)
Well as far as why it works with the HD and not the SSD gets into the drives SATA speed.
The original HD's Apple used in this system was SATA II (3.0 Gb/s), the SSD you put in is a SATA III (6.0 Gb/s) drive.
Think of it this way a Jeep can handle a dirt road without any problems but a Porsche would struggle over the unevenness of the dirt road.
In your case the data feeding down the cable from your SSD is at a higher rate than your cable can handle (road). So you need to replace it to smooth out the road.
So basically, the original cables specs were for SATA II not SATA III drives (or its slightly damaged). The replacement part is now spec'ed at SATA III.
Update (09/01/2015)
Here is the IFIXIT guide to replace the hard drives SATA cable: MacBook Pro 13"" Unibody Mid 2012 Hard Drive Cable Replacement and here is the part you need: MacBook Pro 13"" Unibody (Mid 2012) Hard Drive Cable. Please use this newer cable as some people are still trying to sell the older version."
"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 (Parallel interface). But now, in SATA there isn't!!!
THIRD: The best SSD you can put in your Mac is the original branded Apple SSD, which only can be purchased when you buy your laptop Order To Build... Out of there, you have to buy the most similar component, and sorry, but the only brand of SSD which approach to OEM apple's SSD is Crucial (as far) because of the NAND controller it's fully compatible, Samsung uses a own controller with it could be in some case ,not fully compatible and may cause errors and problems like this.
So please, if the SATA cable works, you DON'T HAVE TO CHANGE!
Thank you very much and i hope this post helps those of you read it!"
diago so that means tha im using macbook pro 2012 i5 ... does ur statement mean that u cant run the 850 evo because of my intel speed ? or what i need to do with that bcause my friend manage to install 850 pro on his 2012 model i7 .... and which means i dont have to change my sata cable because of when putting hdd its boot normally?
i have this problem too, i have 821-1492-01 cable, and i used ssd since 2 years ago, and was ok, but 3 days ago i cant boot, and came folder with question mark. and i think that my ssd broke, and i bought new ssd, i put in , and still came folder with question mark, and i put my hdd back in and OK, can someone tell me the Problem ? should i buy new cable ? i use Macbook Pro 15 mid 2012."
Dan is incorrect, as the laptop's chipset is only SATA II, the drive uses its backwards compatibility to use the SATA II protocol. SATA III is never used by the cable, chipset or drive. It IS possible that the cable has failed, but it has nothing to do with SATA III or the cable not being up to SATA III spec.
