When I boot to Ubuntu on my dualboot system, it appears to boot normally, up to the point when I would normally expect to see the lightDM graphical login screen. However, I see a CLI login screen, which I assume is tty1. Before I get a chance to actually login to it, though, it drops to a black screen with a flashing cursor. Pressing ctrl+alt+f1 does nothing.

When I hit the power button on the laptop, I see the usual scrolling text as Ubuntu shuts down.

I know that this is related to me having installed the proprietary ATI drivers for the video card. Purging the drivers gave me back a GUI, but unfortunately no working desktop environment, and switching to the mesa drivers returned me to the black screen issue.

The only way I can get a stable working CLI is through booting to advanced options and choosing a root shell prompt, as even the failsafe X session doesn't function for me.