Vassar Technology Today | Gone? Think again! How to fully delete data

Matt Leung Assistant Managing Editor

The amount of sensitive and personal data that we create on our computers increases as our reliance on computers increases: letters, embarrassing pictures, stories, financial data, and so on. Most people use folk methods to manage data on our computer, which goes something like this: if you want to delete sensitive data, delete it from the recycle bin, and it’s gone forever.

The major issue here is that data never quite goes away once you create it on your computer. Until you physically melt your hard drive at a high enough temperature, your data is recoverable. When you “permanently delete” data from your computer, such as when you delete it from the recycle bin, the actual data on your hard drive is left untouched. To understand this, we need to take a tour of the operating systems that manage files on your computer.

There are two major parts to file management in operating systems such as Windows and Macintosh. The first part is a file system, an index to keep track of all the files and their physical locations on the disk, like an index of a book; the second part is the actual file that is stored in the hard drive. For convenience, a file is usually split into pieces, or clusters, when it is written onto the tracks, the concentric circles on the disk.

When you permanently delete a file, the operating system only makes changes to the file system and not to the physical data on the disk. For example, in the File Allocation Table (FAT) and New Technology File System (NTFS), file systems used by the Windows family of operating systems, the operating system changes the first letter of the file name in the file system when you permanently delete it. That’s it—your file, according to the operating system, is gone for good! Of course, strangely enough, your data is left untouched and intact on the hard drive. The only difference is that the operating system will allow clusters of the file on the disk to be overwritten by new data in the future. A similar process occurs for Macintosh computers.

One reason for such a seemingly crude process is efficiency. Have you noticed that it might take minutes for a multi-gigabyte file to be written onto your computer, yet it only takes a few seconds for it to be deleted? This is because the operating system does not need to change the file on the disk to delete it.

However, even if the data is overwritten, the data is still recoverable. In fact, data on hard disks need to be overwritten at least seven times in order to be unrecoverable by ordinary means. However, this probably won’t happen anytime soon after you delete your file, because operating systems usually fill up an entire disk before going back to overwrite the used areas. As storage is getting cheaper and hard drives are getting larger, depending on this process to delete your file is not sensible.

An application is needed to ensure deletion of data. The Mac OS X operating system offers this option. When you delete data from the trash, you can select “Secure Erase Trash.” For Windows, numerous applications are available, such as Ontrack DataEraser Personal Edition ($29.95, ontrack.com).

However, the applications mentioned above will only prevent your data from being recovered by ordinary means, such as data recovery applications (ontrack.com sells them as well). Specialized data recovery and forensics laboratories, especially the ones contracted by the FBI, can recover data permanently deleted by the applications mentioned above—no matter how many times the data is overwritten.

The only method that guarantees complete destruction of your data is physical destruction of your hard drive, the method employed by the U.S. Marines.

When a hard disk is heated above its Curie temperature, the temperature at which it becomes nonmagnetic, data will be permanently and truly destroyed. This could mean heating your hard disk to around 1,340 degrees Fahrenheit to delete data that no government agent on earth could recover. Anything less than that, and you are stuck with it.