Logical damage is primarily caused by power outages that prevent file system structures from being completely written to the storage medium, but problems with hardware (especially RAID controllers) and drivers, as well as system crashes, can have the same effect. The result is that the file system is left in an inconsistent state. This can cause a variety of problems, such as strange behavior (e.g., infinitely recursing directories, drives reporting negative amounts of free space), system crashes, or an actual loss of data. Various programs exist to correct these inconsistencies, and most operating systems come with at least a rudimentary repair tool for their native file systems. Linux, for instance, comes with the fsck utility, Mac OS X has Disk Utility and Microsoft Windows provides chkdsk. Third-party utilities such as The Coroners Toolkit and The Sleuth Kit are also available. Even deleted data is also considered to be logically damaged drive for example due to virus attack, if you format the drive or accidental deletion.
Some kinds of logical damage can be mistakenly attributed to physical damage. For instance, when a hard drive's read/write head begins to click, most end-users will associate this with internal physical damage. This is not always the case, however. Sometimes, hard drives can click simply when the drive is not getting enough power - which often occurs on USB-powered drives. Another possibility is that the firmware of the drive or its controller needs to be rebuilt in order to make the data accessible again.
Preventing logical damage
The increased use of journaling file systems, such as NTFS 5.0, ext3, and XFS, is likely to reduce the incidence of logical damage. These file systems can always be "rolled back" to a consistent state, which means that the only data likely to be lost is what was in the drive's cache at the time of the system failure. However, regular system maintenance should still include the use of a consistency checker. This can protect both against bugs in the file system software and latent incompatibilities in the design of the storage hardware. One such incompatibility is the result of the disk controller reporting that file system structures have been saved to the disk when it has not actually occurred. This can often occur if the drive stores data in its write cache, then claims it has been written to the disk. If power is lost, and this data contains file system structures, the file system may be left in an inconsistent state such that the journal itself is damaged or incomplete. One solution to this problem is to use hardware that does not report data as written until it actually is written. Another is using disk controllers equipped with a battery backup so that the waiting data can be written when power is restored. Finally, the entire system can be equipped with a battery backup that may make it possible to keep the system on in such situations, or at least to give enough time to shut down properly.
Two common techniques used to recover data from logical damage are consistency checking and data carving. While most logical damage can be either repaired or worked around using these two techniques, data recovery software can never guarantee that no data loss will occur. For instance, in the FAT file system, when two files claim to share the same allocation unit ("cross-linked"), data loss for one of the files is essentially guaranteed.
The first, consistency checking, involves scanning the logical structure of the disk and checking to make sure that it is consistent with its specification. For instance, in most file systems, a directory must have at least two entries: a dot (.) entry that points to itself, and a dot-dot (..) entry that points to its parent. A file system repair program can read each directory and make sure that these entries exist and point to the correct directories. If they do not, an error message can be printed and the problem corrected. Both chkdsk and fsck work in this fashion. This strategy suffers from two major problems. First, if the file system is sufficiently damaged, the consistency check can fail completely. In this case, the repair program may crash trying to deal with the mangled input, or it may not recognize the drive as having a valid file system at all. The second issue that arises is the disregard for data files. If chkdsk finds a data file to be out of place or unexplainable, it may delete the file without asking. This is done so that the operating system may run smoother, but the files deleted are often important user files which cannot be replaced. Similar issues arise when using system restore disks (often provided with proprietary systems like Dell and Compaq), which restore the operating system by removing the previous installation. This problem can often be avoided by installing the operating system on a separate partition from your user data.
Data Carving is a data recovery technique that allows for data with no file system allocation information to be extracted by identifying sectors and clusters belonging to the file. Data Carving usually searches through raw sectors looking for specific desired file signatures. The fact that there is no allocation information means that the investigator must specify a block size of data to carve out upon finding a matching file signature, or the carving software must infer it from other information on the media. There is a requirement that the beginning of the file still be present and that there is (depending on how common the file signature is) a risk of many false hits. Data carving, also known as file carving, has traditionally required that the files recovered be located in sequential sectors (rather than fragmented) as there is no allocation information to point to fragmented file portions. Recent developments in file carving algorithms have led to tools that can recover files that are fragmented into multiple pieces. Carving tends to be a time and resource intensive operation.