Hello - @Trixar: thks for the flowers... - but to finalize the story and for all newbies like me... of course followed at first the procedure you can read often: to mount the drive on boot you use an entry in etc/fstab. This does not work in this case. It did not really surprised me and I looked into the boot log and found what I assumed: the fstab is worked trough before the ethernet connection is established. So this cannot work. Found a hint by googling in the old SliTaz forum and changed the line after the daemons are loaded in the etc/rcS.conf :
# Daemons to start at boot time. SliTaz only provides a few daemons: firewall,
# Web server (httpd), SSH server (dropbear) and rsyncd, so boot order is
# not really important, but dbus/hald should be started before slim.
RUN_DAEMONS="dbus hald cifs cupsd slim firewall httpd"
# Mount LAN-Drive (NAS)
mount.cifs //192.168.2.60/DATA /mnt/LANPLATTE -o user=GUEST,pass=public,uid=tux,gid=users,rw,nounix,noserverino,nobrl
So this now is working perfect - if the NAS is started a short period before the compi and this isn't a problem at the moment. Of course regarding safety issues (I learned you should work with credentials/hidden(protected)files...) this is not the best way but I'm for the time being interested in the technical solution and will have an eye on these questions later.
Regards Ernst