I specify beforehand that I have absolutely no experience with Apache
Having said that you might want to do the following
Before connecting the external drive
sudo fdisk -l
Repeat the above command after connecting the drive
You will see a new entry in the output (probably at the end). Observe the
device column of this entry. This is the block file that represents your drive. It will be something like /dev/yourdrivesfile .
Now make a folder in
/var/www and then mount the drive on it by:
sudo mount -o defaults /dev/yourdrivesfile /var/www/yourfolder
Before you do this try learning about the mount command and its options, you might need them.
man mount and
info mount will also help.
Changing permissions of your folder to allow
www-data (Apache) to write to it might be something like
sudo chown www-data /var/www/yourfolder, though I 'm not sure about this part.
Hope this helps with the mounting atleast.