I have an old MBP 2011 13" and thought it would be nice to upgrade it to SSD (Sandisk Ultra). I did a fresh install and updated the MBP to Sierra. Now, the main issue is that the laptop sometimes does not detect the SSD and it will show the flashing folder icon. When it does work, it is incredibly slow and will freezes up after the bootup. It is not consistent of why this is happening. Yet, when I replug the old HDD, the computer works fine like nothing ever happened. Then I plug in the SSD into my work desktop and the SSD cannot be detected. At this point, sure maybe it is the SSD's fault... but I have gone through this process with THREE BRAND NEW DIFFERENT SSD and ALL 3 ARE DEAD after installing Sierra. I have also done the Internet Recovery to try and do another fresh install, but it's not going to work if the computer cannot detect the SSD. Something in Sierra is killing my SSD or it's locking up my SSD to not be able to detect it in my computers.