update-grub is setting a wrong linux root partition . How to fix ?

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http://askubuntu.com – Initially, Ubuntu was installed on another partition (sda5 or sda4). At some point I did manually move the root partition to sda2. Now, the problem is that whenever a new kernel is installed, the automatic regeneration of the menu.lst is using the OLD linux partition. So i have to manually change in menu .lst the lines : root (hd0,5) into : root (hd0,1) The question is: how can i configure the update-grub to automatically generate menu.lst with the current linux root ? I assume that the (hd0,5) was written somewhere at installation, but i couldn't find it. (HowTos)