In essence what you need to do is set your system to mount the new HDD at /var/www. This is how I would do it.

Assuming that you have already installed LAMP to your first HDD then you'll want to first work out what your new HDD is. Run

fdisk -l

and see what shows up. Normally your existing HDD should be sda and your second should be sdb (so ensure that those 2 show up). To double check that your primary HDD is sda run

mount

It should should mention sda1 in amoungst some other entries and there should be no mention of sdb. Assuming that is the case mount your new HDD (temporarily) like this

mount /dev/sdb1 /mnt

Now stop Apache and copy across the current contents of /var/www, then rename the current www folder (as a backup), recreate www and create an index file in the existing www so you'll know straight away if something is wrong later down the track...

service apache2 stop
cp -R /var/www/* /mnt
mv -R /var/www /var/www-backup
mkdir /var/www
touch /var/www/index.htm
echo "Error - the drive that contains /var/www on this server is unmounted" >> /var/www/index.htm

Now you can restart Apache to check that your error file works

service apache2 start

Now browse to http://<server-IP> and hopefully you should see the error message above ("Error - the drive that contains /var/www on this server is unmounted").

Assuming that is all good stop apache again

service apache2 stop

In this day and age you should probably do it via the UUID of the HDD - although personally I still use the standard identifiers (e.g. sdb1). You should be able to work out what your new HDD's UUID is with

blkid

Now you want to unmount the drive (from /mnt) and add a new enty to your fstab file

nano /etc/fstab

And add a new line using a format like this: UUID MountPoint FSType Options Dump Fsck
So it will look a little like this

UUID=MyUUID /var/www ext4 defaults 0 0

Obviously subsitute MyUUID for the real UUID (that you discovered with the blkid command)

Save and exit (OTTOMH <Ctrl><X> then <Enter>x2). Now test your new entry with 

mount -a

And then run

mount

to check that your HDD is there. Assuming all is well then restart Apache

service apache2 start

and browse to http://<server-IP> and you should see the default TKL web landing page (or whatever you have replaced it with if you have already started work...)