Data Recovery

Data recovery is defined as the process of salvaging data from damaged, failed, corrupted, or inaccessible secondary storage media when it cannot be accessed normally. Often the data are being salvaged from storage media formats such as hard disk drives, storage tapes, CDs, DVDs, RAID, and other electronics. Recovery may be required due to physical damage to the storage device or logical damage to the file system that prevents it from being mounted by the host operating system.

The most common "data recovery" issue involves an operating system (OS) failure (typically on a single-disk, single-partition, single-OS system), where the goal is to simply copy all wanted files to another disk. This can be easily accomplished with a Live CD, most of which provide a means to 1) mount the system drive, 2) mount and backup disk or media drives, and 3) move the files from the system to the backup with a file manager or optical disc authoring software. Further, such cases can be mitigated by disk partitioning and consistently moving valuable data files to a different partition from the replaceable OS system files.

The second type involves a disk-level failure such as a compromised file system, disk partition, or a hard disk failure —in each of which the data cannot be easily read. Depending on the case, solutions involve repairing the file system, partition table or MBR, or hard disk recovery techniques ranging from software-based recovery of corrupted data to hardware replacement on a physically damaged disk. These last two typically indicate the permanent failure of the disk, thus "recovery" means sufficient repair for a one-time recovery of files.
A third type involves the process of retrieving files that have been deleted from a storage media. Although there is some confusion as to the term, the term "data recovery" may be used to refer to such cases in the context of forensic purposes or spying.

Recovering data from physically-damaged hardware can involve multiple techniques. Some damage can be repaired by replacing parts in the hard disk. This alone may make the disk usable, but there may still be logical damage. A specialized disk-imaging procedure is used to recover every readable bit from the surface. Once this image is acquired and saved on a reliable medium, the image can be safely analyzed for logical damage and will possibly allow for much of the original file system to be reconstructed.

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, and some can produce superior results by recovering data even when the disk cannot be recognized by the operating system's repair utility. Utilities such as TestDisk can be useful for reconstructing corrupted partition tables.

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. Another possibility is that the firmware of the drive or its controller needs to be rebuilt in order to make the data accessible again.

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. This presents the challenge that the beginning of the file is still present and that there is (depending on how common the file signature is) a risk of many false hits. Also, data carving requires that the files recovered be located in sequential sectors (rather than fragmented) as there is no allocation information to point to fragmented file portions. This method can be time and resource intensive.[