Assuming that you have already managed to mount the hard drive manually, the following steps will automatically mount the hard drive every time you boot up your machine.
First, make sure you know which device it is that you want to mount.
run the command:
# cat /etc/mtab
Find you hard disk in the output and copy the entire line into your fstab.
# nano /etc/fstab
*note you don't have to use nano, I personally used gedit.
After this is set and done, reboot and should work. If it fails, just boot up the machine and remove/comment out the line you just pasted into fstab.