MBP Late 2011 freezes under yosemite
So, I installed OS X EL Capitan, (10.11) and i started to transfer a 9Gigs app (xCode) to install it, and I had NO freeze, so it is Yosemite, and fixed in OS X El Capitan :)  It was only a quick test but On Yosemite doing this froze the system every minute. 
If I get one freeze during the next days I'l Edit this post. If not I resolve it.

Anyway, thank you for your help :)

Okay it wasn't OS X but Hardware, The second SSD was installed in the Optical Super Drive bay, and on Late 2011 Mac Book Pro there is an issue on the controller so I installed OS X on the Hard Drive without using the SSD, NO freeze, so I decided to swap SSD and Hard Drive but I don't have the right Screwdriver