iPhoto: Switch between libraries from your computer or an external drive

Background:  You can create multiple photo libraries to organize your photos, albums, slideshows, projects (such as books, cards, and calendars), and then switch between the libraries. For example, you could keep separate libraries for your personal photos and your work photos. Or, if you have a child who takes photos, you can keep his or her photos in a separate library.

Note: You can modify items (such as albums) or edit photos in only one library at a time.

I.  Switch between photo libraries

1. Quit iPhoto.

2. Hold down the Option key as you open iPhoto.

3. Select the library you want to switch to. 

4. Once you locate the second iPhoto library, Select "Choose."

You can create a new library at any time.  To learn how to create a new library, please read:

iPhoto: Create a new photo library