"Performance issues," Shelat says. "It's easier [for the OS] to move those clusters to your free list than to actually write zeros over them. So in order to make those deletes fast, that's the standard way to do it."

Another popular misconception is that formatting a hard drive will permanently rid the hard drive of all stored data. But like deleting files, formatting won't do the trick.

"All the format command does is reconfigure your FAT [file allocation tables] tables at the beginning of the disk and check through the disk to see whether the blocks are still readable," Shelat explained. "But it doesn't do anything to remove or overwrite any of the data. Again, this is for performance reasons."

The file system's efficient behavior means that potentially all of the deleted data on your hard drive is vulnerable to recovery techniques, including the use of easily obtained software that can search a hard drive's sectors for deleted data and recover it. There are also several premium recovery services, such as DriveSavers Data Recovery (http://www.drivesavers.com) and Hard Drive Recovery Group (http://www.harddriverecovery.org), that claim high success rates when it comes to extracting hard drive data. Depending on who you are and what's on your hard drive, snoops can use even more aggressive and expensive techniques to scour your drive, including the use of an MFM (magnetic force microscope), which can even extract overwritten data.



Destroy That Data

Fortunately, securing your hard drive before you sell it or throw it away is surprisingly easy. There are several methods you can use to ensure that all of the data on your drive—whether you previously deleted it or not—won't be recovered by thieves, authorities, or anyone else looking to take advantage of your information.

The U.S. DoD (Department of Defense) outlines standards for hard drive erasing (or "sanitizing") in its National Industry Security Program Operating Manual, which provides guidelines for preventing unauthorized disclosure of classified information. As part of a "clearing and sanitization matrix," the DoD recommends erasing a hard drive by either degaussing with a Type I or Type II degausser; overwriting the drive with random characters; or destroying the drive by disintegrating, incinerating, pulverizing, shredding, or melting. While some of these methods will indeed prevent hard drive data recovery, not all of them are either practical or particularly effective.

A degausser is a machine that removes all of the magnetism from a hard drive, which in turn removes all of the current data and eliminates its ability to hold data. When you degauss a hard drive, it's dead for good. A hard drive degausser can cost thousands of dollars, so degaussing is feasible only for companies, schools, or other organizations that need to wipe many drives on a regular basis. Even for some of those organizations, degaussing isn't the best wiping method because the dead hard drives can't be sold, donated, or otherwise reused after the degaussing process.

Physically destroying your hard drive can be effective if you have the resources to do it, but successful destruction is much tougher than you might imagine. Hard drives are built like tiny fortresses and designed to repel damage, so you can't simply pulverize a drive by, say, throwing it against a brick wall. And sure, melting the drive into a molten blob would obliterate your data, but the same home oven that does a splendid job of baking apple pies can't generate temperatures high enough to melt a drive. Some people believe that drilling holes through a hard drive's platters—the magnetic disks that store the data—can prevent data recovery. Indeed, holes and similar damage to a hard drive can make efforts to retrieve data more difficult, but someone with the right resources can still retrieve data even from a severely damaged hard drive.

For most users, overwriting data is the most efficient method for wiping hard drives. When all of the addressable blocks on a hard drive are filled with new data, typical recovery programs cannot retrieve the data that previously resided in the blocks. One way to overwrite is to fill your hard drive with large files and delete them, but a more efficient method is using any one of several software utilities specifically designed to overwrite hard drive data. These utilities offer a variety of wiping methods that deliver varying amounts of protection, and the method you choose should depend on the level of your data's sensitivity and the time you're willing to spend waiting for the wipe process to complete.

The quickest method included with most utilities is a one-pass wipe that overwrites all drive data with zeros. You might also see methods that blend this simple, zero-character wipe with another pass that adds random characters, or even multiple-pass methods that overwrite with all random characters. Most utilities include the DoD 5220.22-M method, a seven-pass wipe using random characters, complements of characters, and random data streams. Other methods include the seven-pass Schneier method, which uses two passes of specific patterns followed by five passes using a cryptographically secure pseudo-random sequence, and the Gutmann method, which uses a whopping 35 passes, with 27 random-order passes using specific data combined with eight passes using random data.

According to Garfinkel and Shelat, the simplest wiping method included with most wiping utilities provides sufficient sanitization. "If you just overwrite with zeros, even ones, that's sufficient to dissuade all but the most well-heeled people from recovering any of your data," Shelat says.

However, if you're already making an effort to wipe your drive, it's best to use one of the more-advanced wiping methods provided with whichever utility you choose. Although data can theoretically be retrieved with hardware tools from drives either overwritten with zeros or wiped using more advanced techniques, the chances are much slimmer if you use advanced techniques.

If you plan to sell or give away your hard drive, we recommend using a multiple-pass method, such as the Schneier or Gutmann method, to wipe its data. These methods take significantly more time to run than a simple, one-pass zero overwrite, but the peace of mind that comes with these secure methods is worth it. If you need to wipe your current primary hard drive and the utility you're using doesn't offer the option to boot your computer with a floppy disk or CD-ROM, you'll need to install another drive as the primary drive and install the drive you want to wipe as the secondary drive (or just install the drive in another PC). If you plan on throwing the drive away, you can add the extra step of smashing or mangling the drive's connectors, but make sure to wear protective gloves and eyewear before you try this. If you're still paranoid, you can disassemble the drive and sand down the platters, but again, wear protective equipment to avoid injury.



Be Safe, Not Sorry

Even if you don't think your data is valuable, it's still worth the effort to wipe your drive before it leaves your hands. Shelat said that pilfering of hard drive data is probably a common occurrence, especially considering that the process to extract the data is "fairly straightforward and fairly easy." Even if you trust the person buying your PC or hard drive, you don't know where it will end up in the future, when it could hold your old data. You wouldn't sell your wallet with your license and credit cards still intact, so follow the same precautions with your hard drive.

by Christian Perry