Thanks for an insight into modems and kind words.
the installation process only allows GRUB to be installed on MBR and not the partition.
This is not true, but the possibility is hidden
At the last screen of your install you find a sum up of what you've done
In the bottom right corner is the "Advanced" button
Click it and you can select where you want to put grub.
A bug (?) in Ubiquity makes it possible to enter /dev/sdx,x as well as a normal "grubbish" notation, like (hd0,3)
In such a case you have to chainload that grub form something in MBR of course