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The Secure Erase feature allows you to specify how thoroughly the System Optimizer deletes files when it runs.
Normally, when you delete a file, you are moving it to the Trash, where anyone can access it. Even when you empty the Trash, you are not actually deleting the files; you are only deleting the operating system's pointers to the files. The actual data still exists on your hard drive and, unless it is overwritten by other data, it could be resurrected using special recovery tools.