Deleting files may not mean they're gone. Even overwriting them isn't safe. Professor Derek McAuley explains. EXTRA BITS - more on Flash/SSD: http://youtu.be/HQWFCDN9VZI Punch Card Programming: http://youtu.be/KG2M4ttzBnY http://www.facebook.com/computerphile https://twitter.com/computer_phile This video was filmed and edited by Sean Riley. Computer Science at the University of Nottingham: http://bit.ly/nottscomputer Thanks to Richard Faulkner for the photo of the hard disk being destroyed. Computerphile is a sister project to Brady Haran's Numberphile. See the full list of Brady's video projects at: http://bit.ly/bradychannels