1. Erased data can frequently be retrieved through forensics on the magnetic material of a hard-disk drive or backup tape. So-called "magnetoresistive microscopes" have been developed that painstakingly scour magnetic media, and are able to reconstruct the magnetic image of a disk surface. This will show the faint residue of overwritten data. A common security measure is to "wipe" all traces of the data from a machine. Wiping usually involves: Clearing caches and logfiles. Example include browser caches, cookie files, history logs, and recently used document lists. Note that passwords are often stored in cookies and history URLs. Hard-disks "erase" files by simply removing their entries from the directory. The files still exist on the hard-disk. The first step of wiping is to actually erase them by overwriting that area of the disk. Overwriting erased areas of the hard-disk at least 7-times with different bytes (DoD spec) in order to remove all magnetic traces. Forensics specialists can usually read data from a disk that has been overwritten only once. Wiping the pagefile. Most programs do this by repeated allocating all possible memory in the system then freeing it, multiple times. Contrast: Wiping data from the disk is the electronic equivalent of shredding. It is not as strong as degaussing the disk. Misconception: Many crypto programs also contain a feature for wiping free-space. However, deleted files may be overwritten by some other file between wiping passes. This deleted file won't be wiped, and may be recoverable. Key point: In a court case in 2001, the FBI was able to successful retrieve overwritten files from a defendant's hard-drive. History: Microsoft's Windows XP, shipped in October of 2001, contains an automatic wipe feature for permanently deleting files. This has made some forensics investigators worried that they will no longer be able to successfully retrieve deleted files from dumb-criminal's machines. From Hacking-Lexicon

2. Secure file deletion Recovery of supposedly erased data from magnetic media is easier than what many people would like to believe. A technique called Magnetic Force Microscopy (MFM) allows any moderately funded opponent to recover the last two or three layers of data written to disk. Wipe repeatedly writes special patterns to the files to be destroyed, using the fsync() call and/or the O_SYNC bit to force disk access. Homepage: http://gsu.linux.org.tr/wipe/ From Debian 3.0r0 APT