I installed Debian 6 on the same PC where I am running different versions of Ubuntu. Debian 6 installed grub, probably a different version than Ubuntu. I am running update-grub from Ubuntu 10, 11 & 12 and it does not overwrite the Debian boot loader. The Debian grub has failed to correctly define the boot options for 10.04 Ubuntu distro, the other Ubuntu distros where correctly defined. I had to modify the boot options at the grub prompt, intead of single, I wrote recovery nomodeset and I could load 10.04 Ubuntu but not in the recovery mode.
2 Answers
Use sudo grub-install /dev/sdX
X is the drive letter ex:- a, b You can find it out by doing a sudo fdisk -l
Then do a update-grub2

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Recent versions of Debian and Ubuntu both use grub2; both have a version number of 1.99. As indicated by Manula, the problem is probably that the boot loader needs reinstalling. What he doesn't point out is that this is because you probably have multiple versions of th /boot
folder, unless you have this as a separate partition, and hence Ubuntu updates its copy of /boot/grub/grub.cfg
and then Debian uses a different one, which has not been updated.
You might want to look into manually putting the entries you want into /etc/grub.d/40_custom
on both systems, or, if possible, creating a common /boot
partition. Debian and Ubuntu although you then would need to be careful if the kernel version numbers ever get to be the same!

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/boot
folders for Debian and Ubuntu and don't feel confident to put the required menuentry's in/etc/grub.d/40_custom
then you are likely to continue to have problems when switching between Debian and Ubuntu on a dual boot system. – StarNamer Aug 21 '12 at 15:58