Installed a second SSD Samsung 830 256 GB
Smooth, only difficulty was replacing the antenna back. I had to use a bit of force to get the antenna plate in place.
The second sata cable is now pre bended, which is a great pleasure. The attachment in the slot however is pretty lose, you have to be sure it's still in its place before close the mini!
The way to reinstall the software is not very clear.
Take a extra picture with the second harddrive (or SSD) placed!