Skeletons on your hard drive

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Multiple wipes are needed but IMHO hardware changes could resolve this: Multiple wipes are needed on current hard drives to erase the data primarily because:

Each time data is written, the read/write head does not completely polarize the magnetic material, leaving remnants of the previous contents. While it is convenient for computer users to think digitally, in reality each "bit" that is written on the disk drive is stored on a small area of the disk containing many magnetic moments. When a magnetic field is applied when writing data to the disk, most but not all, of the moments are changed to reflect the new data. Under normal circumstances (i.e. not when wiping), the disk doesn't need to change ALL of the moments to represent a bit-- it only needs to change enough of the moments such that the next time the drive wants to read the data, the chance of reading the wrong bit is very low. (For example, the drive might only change 80% of the magnetic moments each time new data is written, leaving 20% of the old data behind.) Of course, for wiping, we would really like to change EVERY moment-- if it is possible to do.

A contributing factor why all of the data is not erased on each write is that the disk drive head does not float over (exactly) the same spot on the disk each time data is read or written. Like bits, many people have a "digital" view of the disk where the head can only be positioned over discrete tracks and sectors. But in reality, the drive is a mechanical (analog) device, where the head can float over any spot on the disk. Each time a specific sector is read or written, the head will generally float in the same area on the disk. But it won't float EXACTLY over the area all the time-- maybe the head will float such that 98% of the sector falls under it each time, but that leaves 2% of the area that it misses each time. That's a small area, but yet leaving 2% of the data behind might be enough for a dedicated organization (like the NSA) to get at your data.

[Note: The numbers I've used above are arbitrary, used as conceptual examples. They're not "real world" numbers from actual drive performance.]

The end result is that multiple passes are needed because each pass doesn't remove all of the data. Writing random bits on the disk is a good idea since it makes piecing the original contents much more difficult-- but it'd be nice if we can get more of the disk erased on each pass, too.

The drive hardware should support a special wiping mode-- hopefully without any significant increase on the cost of the drive. Perhaps the drive can enter wiping mode by setting a special jumper (that can't be changed using software). When the drive is in wiping mode, the drive should:

- Spin the drive motor at a reduced speed. This means that each pass of the drive requires more time, but also that each sector spends more time under the read/write head.
- Increase the power applied to the read/write heard. Increased power means that more of the drive can be erased.
- If possible, increase the "area" of the head so that a wider area of the disk is written. We're in wiping mode, so its okay if the head was so "wide" such that data in adjacent tracks may be written.

The idea is to increase the effectiveness of each wiping pass, without (1) detrimentally affecting the normal performance of the drive, (2) significantly increasing the drive's cost, and (3) avoiding accidents by making sure some hardware change is needed (like setting a jumper) to get the drive in wiping mode.

Cheers!; Posted by: Root User Posted on: 04/20/05 You are currently: a Guest | Members login | Terms of Use