Using advanced options when importing from a hard disk
By default, iPhoto copies photos imported from your hard disk, leaving the originals in their original location and making duplicates for use in the iPhoto library.
However, if you already have an extensive collection of photos saved on your computer and you prefer to leave your photos in their current file structure, you can choose to have iPhoto point to your original photo files instead of copying them in iPhoto. Either way, you can still make full use of iPhoto.