Disk image
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A disk image is a computer file containing the complete contents and structure of a data storage device. The term has been generalized to cover any such file, whether taken from an actual physical storage device or not.
An ordinary backup only backs up the files it can access. Boot information, and files locked by an operating system or being changed at the time, may not be saved. A disk image contains all these, and faithfully replicates the data, so it is commonly used for backing up disks with operating systems, or bootable CD/DVDs.
The image is saved as a file, which for a full CD or system disk may be quite large (~ 10 MB to several GB). This file can be saved onto a hard drive, CD, DVD or other media, for later use.
Disk images can either be compressed using some type of compression algorithm like LZW, or uncompressed ("raw"). Images of CD-ROMs most often carry the file name extension .iso, referring to the ISO 9660 file system commonly used on such discs. The .iso format is the most common format for Linux distributions and other online images. Other common CD image formats are .nrg (Nero Burning ROM's proprietary format), and .bin/.cue.
.cue/.bin
The .cue / .bin format developed by Jeff Arnold for CDRWIN can encode CD Image formats in either 2048 or 2324 bytes per sector. The .bin file is a BINary copy of an entire CD/DVD disc. The BIN file contains all the data stored on the original disc including not only its files and folders but also its system-specifics information, for example, bootable information, volume, volume attributes and any other system-specific data. BIN files are usually bigger than .iso files because in contrast to .iso files they are not a bit for bit copy of the entire CD/DVD.
The .cue file is a datasheet that describes the data stored in the .bin file. The .cue file is in fact a plain text file. A typical .cue file is as follows:
FILE "IMAGE.BIN" BINARY TRACK 01 MODE1/2352 INDEX 01 00:00:00
The file would be saved as IMAGE.CUE
Usage
A common use of disk images is for remote distribution of software such as Linux distributions: installation floppy disks or CD-ROMs can be recorded as disk image files, transferred over the Internet, and the contents of the original disk(s) duplicated exactly by end users with their own floppy or CD-R drives.
Another common use is to provide virtual disk drive space to be used by emulators and virtual machines. This can prevent the CD from getting damaged. It can also reduce bulk when one wishes to carry the contents of the CD along with oneself: one can store disk images to a relatively lightweight storage device which has a higher storage capacity than that of a CD.