During Installation I had to install grub at /dev/sdb manually.
I did it as per description http://wiki.openmediavault.org/index.php?title=Installation
But after installation was finished, the server didn't boot, because /dev/sdb1 wasn't marked bootable.
At next installation attempt, after "execute a shell", I first marked /dev/sdb1 bootable with fdisk an then installed grub manually.