i have got it guys:
on the server side:
1. ftp doesn't follow symlinks outside the user's jail; i forgot it, that is the point; so no symlink, just mount what you want in the user's jail;
2. so say, we have created a user xyz on the server side into /home, and we want to share a usb-hdd mounted on /mnt/sda1;
3. create a directory within /home/xyz with the same name of the usb-hdd which you want to share: sudo mkdir /home/xyz/sda1; give the right permissions for your user xyz;
4. mount your directory: sudo mount /mnt/sda1 /home/xyz/sda1 (i use the options nouser,noauto,umask=002)
5. umount your directory: sudo umount /home/xyz/sda1.
that's it