Recovering from logical (non-hardware) damage

When data have been physically overwritten on a hard disk it is generally assumed that the previous data are no longer possible to recover. In 1996, Peter Gutmann, a computer scientist, presented a paper that suggested overwritten data could be recovered through the use of Scanning transmission electron microscopy. In 2001, he presented another paper on a similar topic. Substantial criticism has followed, primarily dealing with the lack of any concrete examples of significant amounts of overwritten data being recovered. To guard against this type of data recovery, he and Colin Plumb designed the Gutmann method, which is used by several disk scrubbing software packages.

Although Gutmann's theory may be correct, there's no practical evidence that overwritten data can be recovered. Moreover, there are good reasons to think that it cannot.

Corrupt filesystems

In some cases, data on a hard drive can be unreadable due to damage to the file system. In the majority of these cases, at least a portion of the original data can be recovered by repairing the damaged filesystem using specialized data recovery software. This type of data recovery can be performed by knowledgeable end-users as it requires no special physical equipment. However, more serious cases can still require expert intervention.

Online Data Recovery

"Online" or "Remote" data recovery is yet another method to restore the lost or deleted data. It is same as performing the regular software based recoveries except that this kind of recovery is performed over the Internet without physically having the drive or computer in possession. The recovery technician sitting somewhere else gains access to user's computer and complete the recovery job online. In this scenario, the user doesn't have to travel or send the media to anywhere physically.

Although online data recovery is convenient and useful in many cases, it still carries some points making it less popular than the classic data recovery methods. First of all, it requires a stable broadband Internet connection for it to be performed correctly, which many third world countries still lack. Also, it cannot be performed in case of physical damage to media and for such cases, the traditional in-lab recovery has to take place.