Found the reason, because of 'umount -a':

 $ touch $os_mnt/etc/debootstrap/stages/packages

-- OK.

% chroot $os_mnt
# cat /etc/mtab 
/dev/sda6 / ext3 rw,noatime,nodiratime,barrier=1,errors=remount-ro 0 0
proc /proc proc rw,noexec,nosuid,nodev 0 0
sysfs /sys sysfs rw,noexec,nosuid,nodev 0 0
udev /dev tmpfs rw,mode=0755 0 0
devpts /dev/pts devpts rw,noexec,nosuid,gid=5,mode=620 0 0
none /proc proc rw 0 0
# exit

% chroot $os_mnt umount -a
umount: /dev: not mounted

% touch $os_mnt/etc/debootstrap/stages/packages
touch: cannot touch `/mnt/os11/etc/debootstrap/stages/packages': Read-only file
system