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You should take a look at this link, which seems to discuss in a friendly manner some steps to go through to figure out what you can do. The basic problem is your hard drive is getting full and you need to empty up some space. Make sure you've emptied your trash folder, if you use iPhoto check out the trash there (see the first comment on the article) and figure out if there's any files you no longer need, can delete, or can move to an external hard drive.