My Macbook was slow, and then I've wanted to make it faster, it is a Macbook Pro from 2011 or 2012, I can't remember. I bought a Samsung SSD 850 EVO.
Firstly what I did, was to take out the old hard-drive, and then I placed the new SSD in it, I made a bootable thumb-drive, with Yosemite 10.10, and then I tried to boot it from the USB, but I could't install it, because the hard drive didn't show up, because it wasn't formatted. I did that, by taking the old hard drive, inside the Macbook, and used a hard drive dockingstation, to format it, via. disk utility. The hard drive, can only be installed from, the dockingstation, without any issues. I did that, and boot up the Macbook, a Apple logo with a spinning wheel under it, and nothing happens. I tryed everything, used Carbon Copy Cloner, I tried to make a bootable thumb drive, with diskmakerx.
In recovery, when the SSD is in the dockingstation, I was able to install Mountain Lion, from there, I could boot up fine, no issues. After I placed the SSD with a succesfull Mountain Lion, inside the computer, I wasn't able to boot up again, only a Apple logo and a spinning wheel, under it.
I used 4 weeks to make the computer work, I searched all arround Google, Apple.com, and other sites. But I couldn't find any solution.