I was having the same issues with my Evo SSD - The one connected with the original Apple cable is the one being recognized ( correct ) the one with the FIXIT cable is not. In my case, I kept the original hard drive as a backup - what I had to do is use the original cable from apple and connected my Evo ssd with it and use the FIXIT cable with the original Hard drive. I contacted FIXIT with this issues and have not heard a response yet. Several other are having the same issues you are having with the EVO ssd

I just received my Dual kit for my mac mini 2012. I installed the kit per direction provided on here and after completing the transplant ( original 1TB on top and my 500 GB - 820 Evo on the lower ) I power on the unit and it booted up to the apple white screen and the circle icon with a slash in the middle. I read this topic on here before and decided to do it the way they explain it on FIXIT. Well I went back and switched the cable as per several other on here ( The original Hard drive is now using the provided cable by FIxit and the the 820 Evo is using the apple cable ) and it booted up like it should - I went ahead and checked disk utility and everything seems in order.