Disk image

A disk image is a single file or storage device containing the complete contents and structure representing a data storage medium or device, such as a hard drive, tape drives, floppy disk, CD/DVD/BD/HD DVD and key drive, although an image of an optical disc may be referred to as an optical disc image. A disk image is usually created by creating a complete sector-by-sector copy of the source medium and thereby perfectly replicating the structure and contents of a storage device.

Some disk imaging utilities omit unused file space from source media, or compress the disk they represent to reduce storage requirements, though these are typically referred to as archive files, as they are not literally disk images.

Use

History

Disk images were originally used for backup and disk cloning of floppy disk media, where replication or storage of an exact structure was necessary and efficient.

Software downloads

On Macintosh computers running Mac OS X, disk images are now ubiquitous for software downloads. Typically, when a user initiates a download of a software package (often an application or printer/scanner driver), the web browser will download a disk image file (.dmg suffix), which is saved automatically to the user's Downloads folder. Usually the disk image file will then automatically open, creating a mounted volume which appears on the user's Desktop - this volume contains the desired installer or other software. (If the disk image does not automatically open and mount, this can be initiated by double-clicking the disk image file.) Finally, the installer on the mounted volume will often (but not always) auto-launch, and take the user to the start of the installation script, at which point the OS prompts for an administrator password before proceeding with the installation. Once the installation is done, the mounted volume can be "ejected". The disk image file itself can then be deleted, or retained in the Downloads folder (or elsewhere) for future use.

System backup

Some backup programs only back up user files; boot information and files locked by the operating system, such as those in use at the time of the backup, may not be saved on some operating systems. A disk image contains all files, faithfully replicating all data. For this reason, it is also used for backing up CDs and DVDs.

Rapid deployment of clone systems

Large enterprises often need to buy or replace new computer systems in large numbers. Installing operating system and programs in to each of them one by one requires a lot of time and effort and has a significant possibility of human error. Therefore, system administrators use disk imaging to quickly clone the fully-prepared software environment of a reference system. This method saves time and effort and allows administrators to focus on unique distinctions that each system must bear.

Imaging process

Creating a disk image is achieved with a suitable program. Different disk imaging programs have varying capabilities, and may focus on hard drive imaging (including hard drive backup, restore and rollout), or optical media imaging (CD/DVD images).

Hard drive imaging

Hard drive imaging is used in several major application areas:

• Forensic imaging is the process where the entire drive contents are imaged to a file and checksum values are calculated to verify the integrity (in court cases) of the image file (often referred to as a “hash value”). Forensic images are acquired with the use of software tools. (Some hardware cloning tools have added forensic functionality.)
• Drive cloning, as previously mentioned, is typically used to replicate the contents of the hard drive for use in another system. This can typically be done by software-only programs as it typically only requires the cloning of file structure and files themselves.
• Data recovery imaging (like forensic imaging) is the process of imaging every single sector on the source drive to another medium from which required files can be retrieved. In data recovery situations, one cannot rely on the integrity of the file structure and therefore a complete sector copy is mandatory (also similar to forensic imaging). The similarities to forensic imaging end there though. Forensic images are typically acquired using software tools such as EnCase and FTK. However, forensic imaging software tools have significantly limited ability to deal with drives that have hard errors (which is often the case in data recovery and why the drive was submitted for recovery in the first place).

Data recovery imaging must have the ability to pre-configure drives by disabling certain attributes (such as SMART and G-List re-mapping) and the ability to work with unstable drives (drive instability/read instability can be caused by minute mechanical wear and other issues). Data recovery imaging must have the ability to read data from “bad sectors.” Read instability is a major factor when working with drives in operating systems such as Windows. A typical operating system is limited in its ability to deal with drives that take a long time to read. For these reasons, software that relies on the BIOS and operating system to communicate with the hard drive is often unsuccessful in data recovery imaging; separate hardware control of the source hard drive is required to achieve the full spectrum of data recovery imaging. This is due to the fact that the operating system (through the BIOS) has a certain set of protocols or rules for communication with the drive that cannot be violated (such as when the hard drive detects a bad sector). A hard drive’s protocols may not allow “bad” data to be propagated through to the operating system; firmware on the drive may compensate by rereading sectors until checksums, CRCs, or ECCs pass, or use ECC data to recreate damaged data.

Data recovery images may or may not make use of any type of image file. Typically, a data recovery image is performed drive to drive and therefore no image file is required.

Disk Formats

Name	Description
Mac OS Standard	The normal Mac OS disk type.
Mac OS Extended	The normal Mac OS disk type, but with a few added features.
Mac OS Extended (Journaled)	Like Mac OS Extended, but all file operations are recorded.
Mac OS Extended (Case-sensitive)	Like Mac OS Extended, but the disk name is case-sensitive.
MS-DOS (FAT)	Common MS-DOS disk type. Also used as the disk type for many CF and SD cards.

File formats

See also: List of disk imaging software

In most cases, a file format is tied to a particular software package. The software defines and uses its own, often proprietary, image format, though some formats are widely supported by competing products. An exception to proprietary image formats is the ISO image for optical discs, which collectively includes the ISO 9660 and Universal Disk Format (UDF) formats, both defined by open standards. These formats are supported by nearly all optical disc software packages.

See also

• Boot image
• Disk cloning
• List of disk imaging software
• Disk image emulator
• Loop device
• Mini Image
• ROM image
• SDI image
• Mtools

External links

• Back-up-Any-Hard-Drive