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You can copy your iPhoto Library to an external drive and continue to use it there and trash the one on your main drive to free up space or you can work on it on the external drive to clear out what you don't need then copy it back replacing the old one. After you copy it I would check it by using the iPhoto First Aid. Open it once then it become the default, then Quit iPhoto, hold down the Option or ALT (if outside the USA) and Command keys and start iPhoto. Check all the boxes and run Rebuild.