I am wondering if there is any way to have iPhoto reference another hard drive that I have photos stored on. I do not want to import a copy of those photo into iPhoto because of space issues. I do not want to move the iPhoto library either. I want to leave the photos on the drive they are and I will possible be adding another drive that I again want to reference. Is this possible?
Working with referenced photos in iPhoto is possible, but very cumbersome and risky. It is much safer in Aperture (that you can no longer buy, unfortunately) and a bit safer in the new Photos for Mac application.
iPhoto has an option in the Advanced Preferences to leave the photos in their current location, when you import them. It will reference them there. The problem is, that this is error prone. When you accidentally delete or move one of the photos, there is no tool in iPhoto to reconnect the missing original. You could never move them to a different drive, when your external drive fails and you need to replace the drive one day, or when you need to restore the photos from a backup. You cannot consolidate the originals into the library either.
Both Aperture and the new Photos.app support to reconnect missing files and to consolidate in the library. Check out the new Photos, if you want a referenced library. But Photos does not yet support all features of iPhoto.