I'm new to Ubuntu and managed to install version 14 some days ago. That was already really problematic, but somehow worked. I have a GTX970 graphics card, which is not supported by the nouveau driver (AFAIK).
Today I've tried to update to 15.10 and it failed miserably - so my installation is now broken - which is not a big deal - I just want to replace it with a fresh install of 15.10 from a USB stick (created with Univeral USB installer).
Booting from the USB basically works, but the desktop is not useable - the mouse pointer is invisible and many errors show up, so I cannot do the installation.
I've found quite some help on this, but nothing seems to work.
i.e. this answer https://askubuntu.com/a/709131/523935 tells me to try to add nouveau.modeset=0
to the linux command in grub2 - but when I do this, the desktop does not even start up - I end up in a terminal, which asks me to login (to the LiveCD environment). But I don't know which user/password to use (I've tried: "ubuntu"/blank, "ubuntu"/"ubuntu", "linux"/blank, "linux"/"linux", ..)
any ideas?