Using an external hard drive with IPhoto

I have a macbook pro with a large amount of digital photos. I have purchased an Apple external harddrive to backup and store away a mountain of digital photos. I had a help session when I bought the external drive and they showed me how to open IPhoto pointed to the external drive by using Apple button, or control, or shift with a letter key and I can't remember what the command is. It allowed me to open up previous backups and find the photos I wanted. Can you help me out? I looked in the ILife manual and couldn't find it.
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