Is the hard-drive removeable?
If so, you can use pmount without the sudo command to mount it.
To test it, unmount the hard-drive and type in:
Or whatever the device name is called.
Also, the bootscript is executed as root. So no sudo is required.
Scroll all the way down to the bottom, and before the "exit 0" line.
gksu gedit /etc/gdm/PreSession/Default
Put in this:
Change everything in bold to what they should be.
[ "$(ls -A /media/MOUNTPOINT)" ] && echo "device mounted" || mount /dev/DEVICE /media/MOUNTPOINT
Then save the file and reboot.
For the script to work, the folder where your hard-drive mounts in must be empty as a prerequisite.