There have been a number of widely publicized incidents recently of surplus, donated or second-hand hard drives containing valuable corporate information that was left on them when the drives were discarded or sold.
Fortunately, it is fairly easy to make sure drives are completely empty before getting rid of them. Most people know that erasing a hard drive doesn't actually destroy the information. It merely deletes the entries in the file table and marks the sectors as available for reuse. The information can easily be recovered by the use of Norton Utilities or another disk-repair tool.
Simply overwriting the information in the sectors doesn't necessarily render the data unrecoverable, either. Due to a number of factors, ranging from residual magnetic patterns to head repeatability problems, information can often still be pulled from a disk after a single overwrite.
The mechanics of this problem, and one solution, are found in Peter Gutmann's paper, "Secure Deletion of Data from Magnetic and Solid-State Memory" (available at http://www.fish.com/security/secure_del.html , among other places).
The solution is to repeatedly overwrite the disk with specific bit patterns. In his paper, Gutmann outlines one method. The Department of Defense has developed another, similar method, called NISPOM. There are a number of "file wiper" utilities--freeware, shareware and commercial--available that will erase hard disks beyond practical recovery. A number of them are listed at http://www.winapplist.com/security/file_wipers1.htm.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.