I want to mount the public_html to the /hdd2/gallery/
so that /hdd2/gallery/ is treated as public_html for the "website" account.
The above request is a bit contradicting. And even beyond that, you can only mount devices or files with an internal filesystem, but not directories on a device. You rather want to use softlinks, I guess.
1. mount /dev/hdd2 on top of /home/website/public_html/, so that /home/website/public_html/gallery/ has the content
2. make /home/website/public_html/ a softlink to /hdd2/gallery/, so that /home/website/public_html/ leads to /hdd2/gallery/
mount -t ext3 /dev/hdd2 /home/website/public_html/
(will hide original contents of /hdd2/gallery/)
ln -s /hdd2/gallery/ /home/website/public_html/
(must delete public_html directory before creating a softlink with that name)