Click the Shred Files Now button to permanently delete all files in the queue. Click OK to confirm, if you are sure you wish to permanently erase the data. The process takes less than a minute for small files, but can take up to five minutes for large amounts of data.
In time, Windows writes new files over deleted ones -- which still exist on the drive, but are invisible. Shredding free space overwrites all these invisible files on the entire partition -- or entire hard drive -- immediately, making them unrecoverable. To shred free space in File Shredder, click Shred Free Disk Space in the left pane, select a drive and then click Next.
To change the number of passes, select a different algorithm from the Select Secure Algorithm box. One or two passes is usually enough for most people; the overwrite process takes a few minutes. The DoD 5220-22.M algorithm overwrites the data in three passes; three passes provide a good balance between time and security. The seven-passes algorithm and the Guttman algorithm (35 passes) are more secure, but overwriting the data takes significantly longer.