To test how well the system worked, the researchers used a technology called a magnetic force microscopy (MFM), which maps nanometer scale magnetic lines and spots on hard disks that are produced when data is stored. Depending on the orientation of these features, the researchers are able to determine how well their system worked: when a disk holds data, organized lines and spots are clearly visible with MFM; however, after a disk is destroyed, Knotts says, there are no longer such patterns.

The tool has some restrictions. "Imagine taking these systems around in an office," Knotts says. "You don't want stray magnetic fields messing up other hard drives, grabbing stray nuts and bolts, or disrupting pacemakers." To keep the powerful magnetic field from operating outside GuardDog, the researchers developed a shielding based on the same magnets used to erase the disks. They simulated the field on a computer, Knotts explains, and found that small magnets placed outside the main magnet canceled out the field beyond the device.

Although being able to erase data completely is important, it's unclear whether or not this method is necessary, says Simson Garfinkel, an expert on computer and data security (and an occasional contributor to Technology Review). He suggests that an encryption technique programmed into the hardware of a disk, called "crypto shredding," can also completely erase a disk, and do so with the tap of a few keyboard buttons, instead of using a magnetic system. "Why bother with any of that?" he says.

Knotts counters that the government considers some applications to be so sensitive that even encryption is insufficient. And, he adds, for some applications it is crucial to be able to destroy drives during a power failure.

In fact, many ways exist for destroying a hard drive, from encryption to altering its magnetic properties, says Fred Spada, associate research scientist at the Center for Magnetic Recording Research at the University of California, San Diego. The research at GTRI has produced a useful new prototype that could erase information even when an encryption key is unavailable, he says; and it focuses on the standard that his research center is interested in: making sure "there's no chance that there's any magnetic signature that can be recovered by any means."

Currently, L-3 Communications ComCept is looking to commercialize the technology within the next few years. Knotts says the GTRI team developed a miniature version of the device targeted at one-inch disk drives used in high-end cameras and that they are currently working to produce a smaller version of the prototype that would suit laptop hard drives.