Here is, how I solved the problem. Problem was, that grub wasnt installed in /dev/sdb. So I did folowing steps:
First I edited /boot/grub/menu.lst to make sure that system will boot from sdb.
On top of existing stanza I put:
title Debian GNU/Linux, kernel 2.6.8-2-386 (hd1)
kernel /vmlinuz-2.6.8-2-386 root=/dev/md2 ro
then I started grub command shell `grub`
At the 'grub' shell prompt I created new boot records for both drives:
Test went fine this time.
Final step was adding fallback entry, that will automaticlly choose the next item in the menu if the first one fails. In /boot/grub/menu.lst I added entry `fallback 1` just below the `default 0` entry.
Replacement of the simulated failed disk went perfect by Falko's instructions here http://www.howtoforge.com/replacing_..._a_raid1_array
Any comment or better solution is apritiated.