I had an SSD I was using as an external via USB adapter. I knew I was not maximizing the speed of the drive. I also had a 2011 MacBook Pro that could barely handle simple processes. I used the iFixit kit to install the SSD on the laptop and reinstalled the operating system onto that harddrive. Now the computer os lighting fast (even with just 4gb of ram) and has 1tb of internal! Glad I did it.
The repair was challenging, putting back some of the micro screws was like an advanced game of operation. I also slightly unplugged the wifi/bluetooth cable so I had to open the computer back up after finishing the install.
Before screwing the computer lid back on, power up the computer, make sure the drive is showing, AND make sure your wifi and bluetooth still work.