I seem to be having the same problem. I have late 2011 MBP and have bought an iFixit caddy to replace the optical drive and installed a samsung 850 EVO 500gb SSD. The drive runs fine as an external drive using the USB cable, but when it is installed internally it won't allow it to be formatted. Even when it is formatted externally and then installed internally won't allow files to be written to it or install the OS. Some of the answers are saying use another cable or replace the SSD now I am reading that it is data transfer rate issue. The SSD works fine externally. The cable worked fine when it was attached to the optical drive. I bought this combo because the reseller assures me it works for the majority of cases issue-free. Is there a fix for this?
I have solved the problem! At least for my machine. I ended up ringing the local mac people and was told that when they install an SSD optical drive caddy - they put the SSD in the place that the original HD is and put the original in the caddy, because the original architecture can support 6Gb/s whereas the optical drive only supports 3Gb/s - But a standard HD can use the 3Gb/s speed of the optical drive. So I did all that, swapped them over, used the original HD externally to shift all my data etc to the new SSD, installed the original HD in the caddy and hey presto!! I have a new machine that is now FAST! It is running like a new mac. Very impressed. This seems to be the solution. Put the SSD in the original HD space and the original in the caddy. Hope it works for other people.