Cuda 7.5 broke Ubuntu 14.04

Dear all,
I tried to install CUDA on a new ASUS-x550 machine featuring an intel i7-4720 CPU and a single GTX 950m GPU. The operating system is Ubuntu 14.04 and I am using a gnome session with compiz.
I followed the instruction of this website and used the network .deb method. Unfortunatly toward the end of the installation the computer froze, it never happened to me on Ubuntu, I was not even able to access the emergency shell and had to hard reset. Afterword I was unable to boot as I got the error “unable to mount /dev/efi”. I had to reinstall the OS from scratches, I tried again and obtained the same result, with the difference that now I am not even able to boot Ubuntu from an external drive. Luckily the Windows partition boots as normal.
Any advice on how to recover my linux partition and how to install CUDA?
Cheers, Giovanni.