I have just installed Ubuntu 12.04 from a USB alongside Windows 8 in a Toshiba Satellite L855-149, which has a 2GB dedicated AMD Radeon HD 7670M graphics card. 

After I boot and select Ubuntu my screen becomes split and full of stripes. I cannot upload a photo because I have less than 10 reputation. Although I can move the cursor all over the desktop, the system is completely unresponsive to clicking and to commands like **Ctrl**+**Alt**+**T**, even in recovery mode.

I think the problem is that the graphics card has not been properly detected, and that I need to install the appropriate driver manually. But I can't do it because I cannot even open the terminal or anything whatsoever when the Ubuntu desktop has been launched.

Any suggestions as to how to fix this? Also could anybody explain why I do not have this problem while I am running Ubuntu from the flash drive?