I finally decided to upgrade my ubuntu installation after one year. Everything looks great, fancy bootup, wavy-elastic gnome windows and much more. Everything, that one need for effective work. But at least one thing is wrong with installation process. You need to format all partitions mounted as "system" ('/', '/boot', ...). After spending some time hcking ubiquity (validation.py and frontend/gtkui.py) ...
11:55 < cjwatson> validation.py is only used for the old advanced partitioner, which is mostly dead
and finally this helped
11:54 < cjwatson> xyzz: you can remove /lib/partman/check.d/12system_partitions_formatted; just don't even think about filing a bug if it goes wrong :)
So, the solution to bypass this check is:
sudo rm /lib/partman/check.d/*system_partitions_formatted;
