[SOLVED] Trying to mount a partition from a hdd into an lxc debian container. can you guide me because this doesn't seem possible at all or i did this worng.

You don't need to reboot or run systemd command. Just make changes to /etc/fstab and try to mount the directory. My mistake was default should be defaults (with an s at the end). See The fourth field in fstab for more information and options.
oh my.. everything work properly. case closed thanks for your help.
i even did a reboot on everything to be sure everything stay mounted if something bad happen. backup done ! note taken. ty for all the help.