Whereas on a frech Precise install we have :
* grub-pc/install_devices: /dev/vda
grub-pc/install_devices_empty: false
When upgrading from Lucid, the debconf values are wrong since even if grub has been installed, the debconf values do not reflect this so the first stage is not reinstalled, hence the failed boot.
Well, it looks like the culprit could pretty well be a wrong setting of the grub-pc/ install_ devices_ empty variable.
Checking each context I find :
Lucid (initial) : grub-pc/ install_ devices_ empty: true install_ devices_ empty: true
Lucid (post reconf) : grub-pc/
Whereas on a frech Precise install we have : install_ devices: /dev/vda pc/install_ devices_ empty: false
* grub-pc/
grub-
When upgrading from Lucid, the debconf values are wrong since even if grub has been installed, the debconf values do not reflect this so the first stage is not reinstalled, hence the failed boot.