I want to delete all files and folders completely so that users cannot recover the deleted files or folders.
How can I do this?
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I have used BCWipe often over the years. You can try it for 21 days before you buy. It gives you good control over what gets wiped and can also wipe the 'slack space' of files you have deleted in the past, which some wiping tools don't do.
If you don't want to use commercial software, you could try Eraser. It's free and open source. Although I didn't test it myself, it should do the trick.
But you should always be aware, that there can be fragments of the file or the whole file be left on the disk, even if you have erased it securely. A lot of programs save temporary copies of opened files in the temporary folders, for example. So to get safely rid of your data, you would have to find all these locations and delete the files int these places, too. To complicate the matter, they could already be gone, because the programs normally delete the files, when they exit. But of course, they use the file systems deletion method, so the data itself is still on disk, it just doesn't appear in the directory listings any more.