There are many free tools, aka data recovery software, able to make visible again files you have previously deleted, including data that appears to have vanished after emptying the Recycle Bin. Using the keys “Shift +Delete” in Windows to bypass the Recycle Bin will still make those files recoverable, all that Windows does when erasing a file is to delete the records indicating the operating system where the file is located, this makes it invisible to users, but physically, the data, stored on its binary form, still exists.
The only way to make sure that your files have been permanently erased is by overwriting the files you want to get rid of with other data on top by using specialist data wiping software, typical file shredding software will use standard schemes like the US Department of Defense DoD 5220.22-M (3 passes), US Department of Defense DoD 5220.22-M(ECE) (7 passes), Peter Gutmann (35 passes), the latter being more of a gimmick that will consume your time and not a real need, to stop data recovery software used by mom-and-pop shops a single overwrite pass with random data will be enough, to stop expensive hardware recovery tools used by law enforcement agencies and corporations use DoD 5220.22 (7 passes).
You should understand what when you view a document in your computer, Windows will create temporary files of that document all over the operating system, these temporary files consist of back up copies in case of a computer crash or files created for quick access when you run out of RAM memory in the swap file (aka page file), many temporary files are erased by Windows but none of them is securely erased, thus why it is important that good data wiping software includes some form of slack space wiping tool and swap file wiping, when an opponent is unable to recover an overwritten file he will try to discover an old copy in Windows temporary folders or System Restore.
Formatting a hard disk will not erase data
All kind of tracks and confidential data is spread all over your hard disk during the course of its use, like Internet browsing history, if you plan on disposing of the computer, you have no choice but to completely wipe the whole operating system, formatting it is no different from sending the files to the Recycle Bin, to make sure nothing can be recovered everything must be overwritten on top using specialist software.
Some data wiping algorithms fill the last wiping pass with zeroes, aka zeroing the disk, this way you can use an Hex editor to view the hard disk contents, when you do it all that should show is the character “0”, this is proof that the software has done its job properly and no hard disk sector has been skipped, if something else other than a “0” is visible, something has gone wrong.
Sanitizing computer data is time consuming, you can save yourself time encrypting the whole hard disk and all of its contents with something like DiskCryptor, if you are not willing to do it then schedule your data wiping software to automatically wipe your hard disk every day or when you log on/off.
Tools to permanently erase files
- To wipe the whole hard disk including operating system, use DBAN