Data Erasure and Disk Data Recovery

By: Mark Sear

Everyone has probably heard that when you delete data from your hard disk, it is not really deleted, but still exists somewhere on that disk. Even after you have emptied your Recycle Bin, the data still exists. This can be useful when you have accidentally deleted data that you want to retrieve as experts will be able to get your data back. But what if you have deleted sensitive data and you want to make sure that even experts cannot get it back? Do we have to live in fear of our data being recovered when it should have been gone forever?

Let us start with the deleting process and what happens on the hard disk. When a file is deleted from a disk, the actual file contents are not touched at all. All that happens is that the file is marked as deleted. You can easily restore it from the Recycle Bin. When the recycle bin is emptied, the data will still be exactly where it was, but now the space will become available again for re-use. Until the space is overwritten with new data, it is still possible to recover the data from your deleted file.

What other methods of data erasure are there? And do they really ensure that data recovery is impossible?

First, there is degaussing. This involves placing the disk into a moving magnetic field that is strong enough to realign the molecules and remove any data. The down-side of degaussing is that there is not any way to check if the data has actually been totally removed, since the disk will not operate anymore.

There is erasure software on the market to help you get rid of data by writing it over with other data. In the most extreme case, the software will overwrite on every available section of your hard disk and none of the original file system will remain. Only re-partitioning and formatting the disk will make it usable again. More selective erasure applications allow you to overwrite only the 'deleted' files, so that no un-deletion can be done. However, some data might be left. A disk has always more sectors then you are told about. There are always some reserve sectors that are used to replace bad sectors. When a disk suffers an error writing to a disk sector, it will mark this sector as bad sector and it will stop using it. This happens without you noticing it. A defect list entry will be created detailing the bad sector and the sector to use in its place. Any attempt to access the bad sector will actually use the section reallocated from the reserve. Data from these bad sectors might still be recoverable, and there is a remote possibility that it might contain some of your sensitive data. It is all very unlikely, but not impossible.

Is it possible for disk recovery from overwritten disks?
The answer here is very short: No. It is not possible to read data that is overwritten. In the world of "James Bond and Spectre" it might be possible to use electron microscopes to work on infinitesimal differences in recording strength to rebuild data, but this is nothing but a fiction. Even if electron microscopes could be used in this way it would take ages for the reading process to finish. Even if a person lived long enough to finish the reading process, and managed to decode the data, all they would find is data that would by then be long out of date.

When handling with sensitive data, be cautious, follow good sensible procedures, but do not believe in Hollywood science fiction.

Data Recovery
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