iPhoto Library Manager > Help > Storing a library on an external drive
Storing a library on an external drive
You can store an iPhoto library on an external or removable hard drive just the same as you can on your computer's built-in hard drive. To create a new library, follow the normal procedure for creating a new library, and choose the external drive as the location for the library. If you have an existing library that you would like to store on your external drive, use the "Duplicate Library" command in the File menu. You will be given a choice of where to duplicate the library to, at which point you can choose the external hard drive that you'd like to copy the library to. Once you've created or copied your library on the external drive, you can then open it up just like any other iPhoto library.