Using the fstab file, you mount a second HDD at say /mnt/my_data.
Make sure that /mnt/my_data belongs to your user.
Now you want to make it accessible from your users home directory at say /home/user/my_data
# sudo ln -s /mnt/my_data /home/user
The symlink file should be in /home/user, and makes the /mnt/my_data directory and everything below it to appear in /home/user
This can probably also be achieved using your file manager (Konqueror in my case).
Right click on the directory you want actual directory to appear in (/home/user/) and select the option to create a link.
enter the target directory (/mnt/my_data).