It would be worth chrooting into the installation (see HOWTO: chroot from a LiveCD
) to check various things, such as which video drivers are installed, which OpenGL libraries (xorg-x11 or ati) have been symlinked (eselect opengl list), etc. Also, have you had a look at the contents of the xorg.conf file in the HDD installation to see which driver it is specifying? Are any modules specified in /etc/conf.d/modules for loading? Also try removing the "vga=791" from the kernel boot line in the file /boot/grub/grub.cfg while you are chrooting. Obviously make a backup of the file first before editing it.