Now i have found the answer !

simply create rules for udev :

in /etc/udev/rules.d create a file call local.rules
put in it this four lines :

# Add for creatig symlink dvd/cdom and cdrw in /dev
# need some work ie automatique detect
KERNEL="hdc", SYMLINK="cdrom dvd"
KERNEL="hdd", SYMLINK="cdrw"

replace hdc and hdd with your nodes devices ( hda and hdb like you said
in your post )

Don't forget to edit /etc/udev/scripts/make_extra_nodes.sh to remove
(commented) the lines that create the cdrom symlink
like this :

.....
# If we can, add a default /dev/cdrom and /dev/dvd link:
#if /bin/ls -l /dev | grep -w cdrom 1> /dev/null 2> /dev/null ; then
# ( cd $udev_root
# /bin/ls -l * | grep -w cdrom | cut -f 2 -d : | cut -f 2 -d ' ' |
while read optical_device ; do
# It has to be a cdrom. Last one wins.
# ln -sf $optical_device cdrom
# If it's a DVD, set that link as well:
# if grep -i dvd /proc/ide/$optical_device/model 1> /dev/null 2>
/dev/null ; then
# ln -sf $optical_device dvd
# fi
# done
# unset optical_device
# )
#fi
......

This is because the script overwrite your cdrom symlink create by
"local.rules"

Work for me :-)