When you browse the internet you will leave all kind of tracks on your hard disk, if you surf for porn and want to avoid others knowing that, you should be erasing all of your tracks as soon as you have finished.
Your operating system will be making temporary copies all over the hard disk of most pictures and videos you view, even after you have deleted a file, it will still be easily recoverable with specialist computer forensics software.
How many times should you overwrite the data?
The highest security level is usually the Gutmann method, this method will overwrite the data 35 times. This is a complete overkill that will reduce your hard disk life, consume your precious time and will not make you safer than using the US Department of Defense method, DoD 5220.22 M, which overwrites the data 7 times.
US Department of Defense in the clearing and sanitizing standard DoD 5220.22-M recommends the approach "Overwrite all addressable locations with a character, its complement, then a random character and verify".
National Institute of Standards and Technology
"Studies have shown that most of today’s media can be effectively cleared by one overwrite"
2006 NIST Special Publication 800-88 (p. 7)
Commonly used data wiping patterns:
|DoD 5220.22 M||7 passes||YES|
|Gutmann's algorithm||35 passes||Overkill|