Deleting files doesn't actually destroy all the data--it just makes it harder to find. To make sure those files are gone, you need to overwrite them with a tool like Eraser.
The overwriting methods implemented by this free software are based on Peter Gutmann's paper entitled "Secure Deletion of Data from Magnetic and Solid-State Memory," the National Industrial Security Program Operating Manual of the US Department of Defense, and a fast cryptographic random number generator. You can also define your own overwriting methods.
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