Data recovery

Data recovery is the process of salvaging data from damaged, failed, corrupted or inaccessible primary storage media when it cannot be accessed normally. Often the data are being salvaged from storage media formats such as hard disk drive, storage tapes, CDs, DVDs, RAID, and other electronics. This can be 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. Although there is some confusion as to the term, data recovery can also be the process of retrieving and securing deleted information from a storage media for forensic purposes or spying.

Recovering data after physical damage

A wide variety of failures can cause physical damage to storage media. CD-ROMs can have their metallic substrate or dye layer scratched off; hard disks can suffer any of several mechanical failures, such as head crashes and failed motors; tapes can simply break. Physical damage always causes at least some data loss, and in many cases the logical structures of the file system are damaged as well. This causes logical damage that must be dealt with before any files can be salvaged from the failed media.

Most physical damage cannot be repaired by end users. For example, opening a hard disk in a normal environment can allow dust to settle on the surface, causing further damage to the platters and complicating the recovery process. Furthermore, end users generally do not have the hardware or technical expertise required to make these repairs; therefore, costly data recovery companies are consulted to salvage the data. These firms often use Class 100 cleanroom facilities to protect the media while repairs are being made.

Despite this, there are many accounts of users getting a bad disk going long enough to pull their data off, often via slightly bizarre tricks. These include making the drive cold (in the freezer) or spinning it manually on the ground, both actions being used to unstick a jammed platter. Most data recovery professionals recommend against the use of tricks such as these, as they can cause additional physical damage to the drive if done improperly (and in many cases, even when done properly).

Recovery techniques

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, the image can be analyzed for logical damage and will possibly allow for much of the original filesystem to be reconstructed.

Hardware repair

Examples of physical recovery procedures are: removing a damaged PCB (printed circuit board) and replacing it with a matching PCB from a healthy drive (this often entails the movement of a microchip from the original board to the replacement), changing the original damaged read/write head assembly with matching parts from a healthy drive, removing the hard disk platters from the original damaged drive and installing them into a healthy drive, and often a combination of all of these procedures. All of the above described procedures are highly technical in nature and should never be attempted by an untrained individual. All of these procedures will almost certainly void the manufacturer's warranty.

Disk imaging

The extracted raw image can be used to reconstruct usable data after any logical damage has been repaired. Once that is complete, the files may be in usable form although recovery is often incomplete. According to research by the Defense Cyber Crime Institute there are also tools available to law enforcement and government agencies only such as ILook IXimager.

Open source tools such as DCFLdd v1.3.4-1 can usually recover all data, with exception of the physically damaged sectors. (It is important that DCFLdd v1.3.4-1 be installed on a FreeBSD operating system. Studies have shown that the same program installed on a Linux system produces extra "bad sectors", resulting in the loss of information that is actually available.)

Recovering data after logical damage

Far more common than physical damage is logical damage to a file system. 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. Often, either the firmware on the platters or the controller card will instead need to be rebuilt. Once the firmware on either of these two devices is restored, the drive will be back in shape and the data accessible.

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.

Recovery techniques

Two main techniques are used to recover data from logical damage. 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.

Consistency checking

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.

Zero-knowledge analysis

The second technique for file system repair is to assume very little about the state of the file system to be analyzed, and using any hints that any undamaged file system structures might provide, rebuild the file system from scratch. This strategy involves scanning the entire drive and making note of all file system structures and possible file boundaries, then trying to match what was located to the specifications of a working file system. Some third-party programs use this technique, which is notably slower than consistency checking. It can, however, recover data even when the logical structures are almost completely destroyed. This technique generally does not repair the underlying file system, but merely allows for data to be extracted from it to another storage device.

Recovering overwritten data

Further information: Data remanence

When data has been physically overwritten on a hard disk it is generally assumed that the previous data is no longer possible to recover. In 1996, Peter Gutmann, a respected 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.

Tools

Boot media

It is often the case that data recovery and forensics operations cannot be done on a running system. As a result, it is common to use a specialized boot disk, Live CD, Live USB, or any other type of LiveDistro containing a minimal operating system and a set of repair tools. When floppy drives were still common, the boot disk was typically a very minimal LiveDistro on a floppy disk (such as the Mac OS Classic Disk Tools disk, standard with every system release). However, as operating system complexity has increased, it has become more common for developers to include recovery tools on the same media as the OS installer. There are also many purpose-built LiveDistros that include advanced data recovery and forensics tools.

Main article: List of LiveDistros

Knoppix

The original Linux LiveCD. It contains many useful utilities for data recovery.

Ubuntu Rescue Remix

A GNU/Linux live CD that includes free-libre, open source data recovery and forensics tools.

Specialized software

Consistency checkers

CHKDSK

A consistency checker for DOS and Windows systems.

Disk First Aid

A consistency checker for Mac OS 9.

Disk Utility

A consistency checker for Mac OS X.

fsck

A consistency checker for UNIX filesystems.

File recovery tools

Power Data Recovery

A read only tool to recover data after accidental deletion, format, re-partition or system crash.

Foremost

A tool for extracting files from a disk image.

NTFSprogs

Can be used to recover files from NTFS partitions.

PhotoRec

A file recovery tool designed to extract image files (photos) from failed storage devices including hard disks and digital cameras.

EASEUS Data Recovery Wizard

The professional level, most cost-effective and ease-to-use software available for data recovery.

EASEUS Data Recovery Wizard professional

Highly effective and complex data recovery software for non-destructive data recovery from hard disks.

Forensic toolkits

The Coroner's Toolkit

A suite of utilities aimed at assisting in forensic analysis of a UNIX system after a break-in.

The Sleuth Kit

A suite of forensic analysis tools for UNIX, Linux and Windows systems. Includes the Autopsy forensic browser.

Imaging tools

Main article: Disk image

dd (Unix)

The classic UNIX tool for dumping data.

dd_rescue

An open source dd-based utility for data recovery scenarios, by Kurt Garloff.

GNU ddrescue

Another open source utility based on dd, part of the GNU project. Semi-automated, and includes more features than similar dd_rescue.

DCFLdd

An imaging tool for FreeBSD.

ILook IXimager

A tool only available to governmental agencies.

SpinRite

A well known imager written in x86 assembly language.

DeepSpar Disk Imager

Hardware/software imaging solution for professional data recovery companies.

Partition recovery tools

Main article: List of partition utilities

Parted

A program for creating, destroying, resizing, checking, and copying partitions.

TestDisk

A utility designed to recover lost or damaged partitions on a wide variety of systems.

See also

• Data loss
• Backup
• Continuous Data Protection
• Error detection and correction
• Undeletion

Further reading

• Tanenbaum, A. & Woodhull, A. S. (1997). Operating Systems: Design And Implementation, 2nd ed. New York: Prentice Hall.
• "Disk Imaging: A Vital Step in Data Recovery"- This white paper describes disk-level issues that must be handled by any disk imaging tool intended for data recovery.
• Goutam Kumar Saha, Software Implemented Fault Tolerance through Data Error Recovery, ACM Ubiquity, Vol. 6(35), 2005, ACM Press, USA.
• Goutam Kumar Saha, Software Based Fault Tolerant Array, IEEE Potentials, Vol. 25(1), 2006, IEEE Press, USA.

External links

• Data Recovery and protection strategies - A guide from Microsoft on Data Recovery and best practices and procedures
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