Deleting data from magnetic disks

Sanitising data media is not easy. At the end of its useful life, or whenever you plan to pass a computer on, it is vital to ‘scrub’ the data off the hard disk. This is a task to be taken seriously.

The threat

The new recycling rules in Europe impose costs on those wanting to dispose of computers, so it is economically attractive to send computers overseas. The un-encrypted data on hard disks provides an attractive target for industrial espionage and those wanting to steal others people's identity. Are your bank details on the computer?

Public guidelines on the subject are open to criticism. If you subscribe to conspiracy theories you would say the police and intelligence agencies might be guilty of deliberately understating the problem. Big encryption keys and heightened security awareness make the eavesdropping part of these agencies snooping job harder.

Why is deleting so difficult?

If you ever bother to examine the graphics in computer books explaining how digits are stored on magnetic media, you see a nice neat on/off, square waveform. In reality, the ‘marks’ made on a magnetic disk look more like the dent you might make in some damp sand. The edges are blurred so they overlap each other.

Now imagine making a new mark between two existing dints in your sand pit and you will realise that the new dips affect the existing ones. It is all a lot less precise than the pictures suggest and this is where the problems with ‘after images’ begin. The technology is just too clever.

Before you lose faith in your hard disk and rush to do the long overdue backup do not worry. The system works amazingly well. It sorts out the mass of magnetic impressions with some complex encoding of the data which allows the recording to be replayed even though the data has errors. Some very clever maths allows these errors to be identified and corrected. The result is a robust, accurate and durable way of storing data.

When you start to think about the write process, the head has to move into position and it might overshoot, depending which direction it arrived from. So the head might not be exactly on target. Then imagine what happens when the head has to write a series of 1s where there were 0s before. You now have a .90 whereas overwriting a 1 might yield a 1.10. Because the system is digital, that is on or off, .9 or 1.1 can both be read as 1.

The problem

The point is that simply wiping the data will not completely hide the underlying information. And, thanks to that wonderful encoding system we mentioned, by employing some clever technology to scan the disk (known as Magnetic Force Microscopy - MFM) a good image of your overwritten data can be produced.

These scanners do not simply look along the track where the data was laid down but at the marks made to the side of the main track. Remember the sand analogy used earlier. The edges of the dots in the sand can still give good information although it may overlap with adjacent tracks. This modulated data can still be interpreted.

Few people now labour under the delusion that erasing a file removes it. In most computers, all that happens when you delete a file is that the pointer to the file is removed and the file now points to the recycle bin. The file is still there. It can be recovered in seconds.

Even reformatting the disk does not necessarily remove data. Formatting remakes the directory and restores the index makers on the disk but the data is normally left untouched. For an expert, recovery is simple.

In the olden days we used a programme called ‘delZ’ to delete data securely which wrote a pattern of Zs through the disk, not once but three times. There are some clever products that do something similar and a few links to free software are given below. This really is the only way to render the contents of a disk unreadable. The disk must be constantly re-written with random rubbish.

The military rely on removable disks which can be treated like documents, except that they cannot be fed through the shredder. Ferric chloride does a great job of destroying the surface of the disk once you have access to it.

The DIY approach to hit things with a hammer or to drill a hole in the case will deter amateurs, but if the data is commercially sensitive there might be someone out there who will pay to have your disk read. Neither of these approaches work. The modern disk drive is remarkably robust. The data has to b destroyed down at the surface of the disk itself.

And on the subject of durability, data that has been written and left in a nice warm environment can ‘sink’ into the disk and become even harder to erase. That is great for the system software where durability is a bonus, but not so grand for your banking details. A nice warm computer almost welds the data to the disk over the years.

So

The moral has to be to take great care of your hard disk if you get rid of an old computer. If there is data on the disk that would compromise you, do not dispose of the disk inside the computer. Why not think about transferring the disk to you new machine.

This is all fine for magnetic media but none of it will work for optical disks. That is another story….

Some sites to check - These provide the only safe way to securely delete data on your disks

MAC

Unix

And most of the PC products have Unix versions

© c jones 2006

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