I came here after looking at your other thread
First off. If sticking with Ubuntu. Have you tried booting using the safe graphics option in grubs menu?
Second. Forgive my ignorance on a
good 'ole Matrox MGA Millenium card
I run cutting edge Debian operating systems like AntiX. I have noticed that some of the newer kernels in Squeeze and Sid are dropping support/drivers for older gear. My suggestion maybe is since you are familiar with Mandrake already. You might go with a rpm based distro like Mandriva or CentOS (free redhat) or Fedora or OpenSuse.
Just because Ubuntu works on a lot of Hardware and Is popular. Does not mean it is the only good Debian Fork out there. Debian Lenny, AntiX, Mepis, Sidux (cutting edge), Knoppix, are good live Cd oprating systems based on Debian that you could burn a cd and run live and see how well they work on your gear also.
I would try in Ubuntu to boot into safe graphics mode first which is vesa mode. Then look for the appropriate driver for your card from there. Another alternative would be to unplug your moniter from the
Matrox MGA Millenium card
and plug it into the motherboard vga plug and see if Ubuntu boots up from there..
Usually when dropped to a grub prompt. You can boot up the live cd. Go to /boot/grub/menu.lst on your install. Look at the kernel line in the menu list and enter appropiate kernel information into the grub prompt. Trouble is the Ubuntus you are using use Grub 2 which I am not familiar with yet. I use Grub 1.5 legacy whenever possible and Ext3 file system when ever possible since those are what I am used to using. So I won't be of any help there.
Like how you mounted Kubuntu. I would have just mounted it to root also / like you did with Ubuntu. It has it's own partition like ubuntu on /dev/sda2.
I don't get
Kubuntu mounted at /mnt/Ubuntu_dsktop_91
why that was done at all. Simple is sometimes always the best way to go when learning.
Happy Trails, Rok
Edit: I see reed 9 has a grip on this. He is familiar with grub 2.