This is the second Data Doubler that I have installed. The first installation in my own mini was a bit of a struggle. I assembled my first pc in 1976 so I have a lot of DIY experience - my data processing and computer experience goes much further back (1946). There were a couple of steps in the installation instructions of about three years ago that could have been clearer - mainly for some reassembly steps. None the less, once the problems were solved, the computer has functioned flawlessly.
The second installation was in my wife's mini - so a SSD and a larger capacity HD could be added. The instructions have been updated and are much more helpful and instructive. It took me perhaps an hour for the whole process including formatting the drives and upgrading the OS.
The resulting improved performance is remarkable for both units!
Three suggestions: 1. When reconnecting the power supply, leave as wide a gap as possible when reinserting the main board. Then this critical reconnection is much easier and surer. 2. When reinserting the top drive, stand the mini on its top face and tilt so it is maybe 20 degrees beyond vertical. The pins on the drive will fall into place almost without any struggle. It also helps to make two pencil marks on the top front of the case to mark the localtion of the holes for the pins to line up the drive as it is inserted. 3. When reinstalling the antenna assembly, the edge of the perforated screen must rest on a very narrow ledge on the case. If any part of the screen is below the ledge, the screw holes will not line up.