When you delete files on your computer, remember that they are not really gone. Pressing the “delete” button on a file or folder simply removes it to the recycle bin. Emptying out the recycle bin does not delete the file but just tells the computer that the digital space the file was using can be used. However, with simple software tools the “deleted” files can be removed. Only when the data is overwritten is the file really “deleted”.
The NY Times has a GREAT overview of file deletion programs and writes There are, however, several options for securely eliminating data from hard disks, U.S.B. flash drives and other storage media. These programs overwrite data with meaningless characters to render it unrecoverable with today’s data recovery techniques. Some of the programs can overwrite entire drives, while others can single out individual files or other information saved by a computer’s operating system or programs like Web browsers. Shredding machines that can destroy diskettes, CD’s and DVD’s are also available.
Several programs are available to overwrite entire hard disks. These programs wipe away everything, including programs, documents and the computer’s operating system, so users need to be sure to have backup copies of important data and software before using them.