Do you copy them direct to the hard drive or do you use iPhoto / Aperture? If you use iPhoto or Aperture, open the program while holding the option (alt) key, then choose 'create new library', choose the 750GB hard drive as target and klick ok. Then, just drag the photos into the iPhoto / Aperture window and you're done.
If you drag them directly onto the HDD, just choose it in the finder and you're fine.