The hard drive in my MacBook Pro is at full capacity. Luckily, I have found out that Aperture has a functionality to move existing original photos stored in my hard drive to an external drive or drives while keeping an index view of these distributed photos in the Library of Aperture. The following is a way how to do it in Aperture 3.
1. Choose Projects view under LIBRARY on the left side of your screen.
2. Click into the Project where you store the photos you want to move to an external drive
3. Select the photos you want to move
4. On the top of the screen, choose File => Relocate Originals
5. Make sure your external drive is connected to your computer / laptop.
6. Now, an new Aperture window comes up. This is an opportunity for you to organize. Choose the external drive under DEVICES on left side of window.
7. If needed, create a new folder by clicking New Folder button in the bottom left of the Aperture window. I created my folder called "Photos (Aperture)".
8. I wanted all the photos in the Project to be contained neatly a sub-folder within the new folder Photos (Aperture) inside the external hard drive, so I selected Project Name in the Subfolder Format drop-down menu. Then click Relocate Originals button.
9. A progress bar appears showing the file being moved
10. Once complete, you will see that there is an icon on the right bottom of the photos moved.