Solid-state disks offer 'fast erase' features

Military-grade SSDs are easier to erase, although harder to restore

As the pilot ejects inside enemy territory, the fighter jet triggers an automatic data-destruction sequence. Within 15 seconds, the highly classified mission data on the solid-state disk has been wiped out.

The storage device in this scenario didn't just burn up like the voice recorder in Mission: Impossible. Instead, the system's manufacturers simply took advantage of a key property of the flash memory chips that make up solid-state disks: Data can be erased much more quickly and thoroughly than it can with a magnetic, spinning hard disk. Solid-state disks, or SSDs, don't require six or seven passes to erase all traces of the bits on every track and sector. Once the bits have been reset in every flash memory cell, that data is gone forever, although meeting the most stringent government disk-sanitization requirements may still involve two or more passes.

The process is quick and efficient. "You're talking about seconds," says Gary Drossel, vice president of marketing at SiliconSystems Inc., a manufacturer of SSDs used in government systems. With a typical hard disk, just the process of getting every block on a drive of that size to spin under the read/write head would take almost an hour and a half, and the entire process could take three to four hours on a fast eSATA drive, according to experts at Texas Memory Systems Inc. and Kroll Ontrack Inc.

How Clean is Clean Enough?

Hard disks can leave behind a magnetic residue that, theoretically -- because of something called the hysteresis effect -- could be used to reconstruct the hard disk data. Whether in fact that's practical with today's disk drives is a matter of debate. But does this same potential weakness apply to solid-state drives? No, says Jamon Bowen, enterprise architect at enterprise SSD maker Texas Memory Systems. If normal disposal processes are followed, your SSD data will be irretrievably erased, he says.

"With NAND [flash], you're storing a full amount of electrons on a floating gate, so there's no real way of telling what the value of that transistor used to be. Once you fully erase the drive, there is no ability to recreate the data," says Drossel.

"To my knowledge, a reprogrammed cell does not contain any previous data," says Sean Barry, senior data recovery engineer at Kroll Ontrack Inc. But not everyone agrees. BitMicro Networks Inc. claims that a hysteresis effect can also affect SSDs and has created a hardware-level secure-erasure function to deal with that. For all practical purposes, however, if data-recovery firms can't get the data back after a single secure-erase pass, most nongovernment users needn't worry about it.

Assuming, that is, that the erasure pass got everything. Unfortunately, some cells can be missed during the process. SSDs have extra memory cells beyond what is allocated to the file system. These are used by a "wear-leveling" feature that distributes data across that larger area to extend the life of the SSD. Those cells may be swapped in and out of the area used by the file system.

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Robert L. Mitchell

Computerworld
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