Sometimes my Ubuntu gets stuck with purple screen, and the only way to get out of that is a reboot. I have enabled verbose mode in grub2, but I see nothing on the screen.
Or sometimes in low resolution mode.
However, sometimes it boots correctly and prints all the things at startup.
And sometimes it gets stuck after printing "detecting usb number 3..."
My system is a Lenovo u410 with Ubuntu 12.04. I have installed bumblebee, and it works fine (I tried it with optirun). I have checked the pci devices, and both my graphic cards are known. I think there is a problem with my usb 3.0