Review of several implementations of information destruction from magnetic media..
A review of several implementations of information destruction from magnetic media.
In this popular science study, we will try to evaluate the potential capabilities and effectiveness of several approaches to erasing information from magnetic media, in particular from hard disk drives.
Let's recall what a hard disk is and how it works.
From the very first models to the present day, a hard drive consists of magnetic disks (or «pancakes») on which information is written, a magnetic head that writes and reads information from the disks, and a control controller. Erasing information, if we mean guaranteed erasure without the possibility of recovery, is an effect on the magnetic plates themselves. Effects on the heads or controller have nothing to do with erasing — their damage (destruction) will lead to the inoperability of the device in normal mode. However, a fairly simple procedure for rearranging the «pancakes» into a previously operational hard drive of a similar type will allow you to successfully read all the information.
We will try to consider the pros and cons of real data destruction devices available on the market.
Efficient (reliable) erasing methods come down to remagnetization (demagnetization) of disks or their physical destruction.
Let's start with the latter. The main types of physical destroyers are shredders, presses and punches.
A shredder simply grinds a hard drive into fragments from which it is practically impossible to restore anything. However, the hard drive case is a fairly strong aluminum structure that very effectively resists mechanical damage.
Therefore, shredders are quite bulky and energy-intensive to operate. They can only be used as a stand-alone device for mass media recycling.
Press – Crushes the hard drive, critically deforming the pancakes.
The reliability of destruction (improbability of recovery) is worse than that of shredders — the information remains on the pancake, however, its reading is very difficult. Presses also have all the disadvantages of shredders.
Punches are mechanical devices that disable a disk by punching it through in one or more places. However, the information remains physically recorded and can be restored from the entire volume of the disk, except for the punched hole. Nevertheless, their efficiency is quite high, and their design solutions allow them to be used not only as external devices, but also to be built into servers, if necessary, destroying information even from a working hard disk.
Despite the apparent effectiveness of the result, mechanical destroyers are bulky and their use is highly specialized — mainly the disposal of disks removed from the computer. A mechanical punch can fail, units that are not moving for a long time can simply oxidize, get dirty and not work at the right moment, a more durable hard drive case may not be punched, etc. And the main disadvantage is that the information continues to remain on the disks, although its extraction is quite difficult.
More reliable are hard drive magnetization reversal devices that do not aim to damage the disk — they actually erase information. The devices are widely represented on the specialized market in Russia, as well as around the world (English name — DEGAUSSER — demagnetizer). These devices use either permanent supermagnets or an electromagnetic effect for magnetization.
Devices on permanent magnets are used only for the disposal of media. Their magnetic field is constant, and they cannot be combined with a working disk.
From the point of view of fast erasing of information, the most interesting are electromagnetic destroyers, which can generate strong magnetic fields at any time, have a compact enough design for use with working disks and even be built into working computers — servers.
All these devices operate in accordance with the laws of physics: an electric current passed through a conductor closed in a ring (star, rectangle) creates a magnetic field of a certain configuration around this conductor. If you place such a conductor in a certain way in close proximity to the drive and create a current of sufficient strength in it, the induced magnetic field is guaranteed to remagnetize the disk pancakes, erasing all information.
In practice, 2 types of magnetic field emitters are used
1. flat — in which the magnetic field is created by a flat spiral. The hard drive is located next to the emitter.
2. Volumetric, or solenoid — the hard drive is placed inside it.
Let's try to consider the efficiency of each type of emitter from the point of view of mechanical design and physics of the process.
Flat emitter.
1. Design. The emitter has a finished shape, usually a rectangle of insignificant thickness (0.5-2 cm), and can be placed close to the disk in the plane of rotation of the pancakes. Sometimes the emitter is installed between two disks, since the magnetic field spreads in both directions from the plane of the emitter. Theoretically, in the latter case, both disks should be erased. The method is convenient, for example, when used in servers and data arrays, when an emitter is installed instead of disks, without redesigning the server. The disks to be erased are in regular places, cooling, server interfaces are used.
2. Physics of the process. The distribution and power of the magnetic field of the spiral are shown in the figure. (side view, for clarity. Lilac — emitter, black — hard drive with two pancakes)
A feature of the magnetic field of the spiral at each point in space is a very strong nonlinear dependence of its power on the distance to the spiral and the distance from the center — the further, the weaker.
If the field has a value of x in the middle of the emitter, then halfway to the edge of the emitter it will be x/2, and at the edges – x/6 or less. The same situation is at a distance from the emitter plane – the field at a distance of 1 centimeter weakens by 30-40%. This effect gives an extremely uneven distribution of the field on the hard disk pancakes, and erasure in the zones closer to the center of the emitter is complete, and in the zones above the edge of the emitter – partial or ineffective. In a hard disk, the distance from the emitter plane to the information plates is determined by the presence of the controller board and the thickness of the chassis case, on the other hand – usually a thin protective cover, that is, it matters on which side the emitter is installed. Positioning the emitter relative to the center of the disk plates is also very important.
Fig. Effective erasure zone of a flat emitter
In this case, the peak of the magnetic field power will fall on the spindle (motor) of the disk, and therefore will be wasted. It should be taken into account that in disks with more than one plate, the plate next to the emitter is even further away, and accordingly the effect of the field is weaker. The situation is somewhat improved if you use 2 emitters, on both sides of the disk (see fig.). In this case, both disk plates receive a relatively equal magnetic effect, and the factor of which side of the disk (controller or casing) the emitter is installed on is removed. However, a significant weakening of the erasing effect towards the edge of the pancakes remains. Nevertheless, a hard disk erased by a flat emitter is guaranteed not to be able to operate in normal mode, most of the information is irretrievably erased. The emitter is easy to install — it is simply placed above (under) the hard disk, or (in the case of disk arrays) is installed instead of the hard disk.
Volumetric emitter — solenoid.
1. Design. The emitter is a hollow frame into which a hard disk is inserted. In addition to the disk, interface cables must be connected, and forced cooling must be provided. The emitter with the disk takes up approximately a 5-inch bay (CD-ROM) and requires additional fastening for installation in the server case, or removal together with the disk to a separate case, with the power and interface cable coming from the native server. In this case, the donor server does not require any modifications, the native disk baskets can be used in normal mode. One destruction device can contain disks from several servers.
2. 
Physics of the process. The distribution and power of the solenoid magnetic field are shown in the figure. (side view, for clarity. Purple is the emitter, black is the hard drive with two pancakes)
The peculiarity of the magnetic field is its uniformity at any point inside the solenoid (the field power decreases rapidly only at the edges of the emitter). As a result, information is effectively erased from the plates at any point of the pancake. However, the emitter has much larger dimensions (compared to a flat one). This method of erasing is the most effective and provides a guaranteed erasure of all information (provided that the emitter creates a field of the required power).
Fig. Effective erasure zone of the emitter solenoid
Instead of conclusions.
From the above it follows that there are many methods of erasing (destroying) information, as well as devices that implement them. Each has its pros and cons. In particular, mechanical methods should be used for mass destruction of disks, and electromagnetic methods for emergency erasure from working disks. If you need to save as much as possible on equipment at the expense of reliability, you can choose a flat two-sided emitter; if you need maximum erasure efficiency and versatility, you can choose a volumetric emitter.
That is, you need to choose the method and implementation based solely on the conditions of the task and the goals that are set for the destruction device.