Comment 7 for bug 508173

I may be able to figure the bug out without this, but an upgrade log with DEBCONF_DEBUG=developer set in the environment would be helpful. Cursory examination doesn't show a code path for which this is possible on upgrades (the intent is that if you're upgrading from something not explicitly understood, then you'll get a question; although I should also enhance the "explicitly understood" bit to cover the case of installing to a partition on a single-disk system like this), but perhaps I've missed something.

Out of interest, why are you installing GRUB to a partition? This is unreliable in some quite fundamental ways and if you can manage to avoid doing this it would be a good idea.