sdb9 is where mint's /boot/grub/menu.lst is located.
OK.. if sdb9 is where your Mint's menu.lst is, you would tell the installer to install the grub kernel to that partition. That would be:
to the installer. The default is hd0 which is the sda mbr.
If you put in hd1,8 you will not disturb the mbr that you are booting from.