You need to install udev-oxnas in chroot. Type sync and hit enter It makes sure that everything is written into filesystem.
Nokia-N900:/# pacman -S udev-oxnas resolving dependencies... looking for inter-conflicts... :: udev-oxnas and udev are in conflict. Remove udev? [y/N] y error: failed to prepare transaction (could not satisfy dependencies) :: mkinitcpio: requires udev>=177-1