xavier0912, try this:
Boot Ubuntu, open a terminal and type
This will install GRUB on the MBR of sda, and direct it to boot sdb2 (the boot partition).
grub> root (hd1,1)
grub> setup (hd0)
While still at the grub> prompt, next type
This installs GRUB on the sdb6 partition (your Jaunty installation).
grub> root (hd1,5)
grub> setup (hd1,5)
You need this so that when you use the "chainloader +1" command, GRUB on the MBR will hand-off to GRUB on sdb6 to boot Jaunty. For each additional Linux distro that you install, you'll need to install GRUB (using the "root (hdX,Y)" and "setup (hdX,Y)" commands) on that partition too.
Now quit grub:Next, at the terminal prompt, type
This should open the menu.lst on sdb2 (the boot partition).
sudo mount /dev/sdb2 /mnt
gksu gedit /mnt/boot/grub/menu.lst
since you want to boot sdb6, not sdb2.
Save and exit gedit. Reboot. Let us know how it goes.