Make sure that your computer supports the connection used by the external hard drive, check this out.
http://www.kenstone.net/fcp_homepage/partitioning_tiger.html
How to move your iPhoto library to an external hard drive (Mac)
1. Find your iPhoto library which will probably be in the default Home folder, (user name, pictures, iPhoto library.) It’s a large file with all of your pictures in. Connect the hard drive to your computer, and then drag the library to the relevant place on the drive, probably inside a named folder like Pictures.
2. After it has copied start up iPhoto and at the same time hold the alt key down, then a box will ask you to choose a library. Select this option and then find the library that you just copied to the external drive. Now it should load from that drive and remember these settings.
This will de clutter your drive and is great for Mac laptop