Original post by: sven ,
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?