A disk image is a block-by-block copy of all the data on a particular part of a hard disk. Imaging programs ignore file systems, boot sectors and most of the other structure on the disk to create a copy of everything. Even open files, hidden partitions and boot sectors, which are ignored by the operating system's copy command, are copied into the image.
The most common use of disk imaging is to provide working backups with fast recovery to handle the most common kinds of storage problems, such as accidentally deleted files. These programs typically take images of changed files at frequent intervals and can recover files and folders quickly -- often without the aid of storage administrators.
The other use of disk imaging is to create an image of an entire disk or array for disaster recovery purposes. Often disk imaging software will export the image over a network to a tape library or remote location for additional protection. While many products will do both jobs, especially in the SMB arena, the software tends to be specialized for one job or the other. While you probably have working backup imaging capability in your existing software, don't automatically assume it will do a good job for disaster recovery. It is worth taking a look at the feature set in what you have and perhaps purchasing a program (or subscribing to a service) for disaster recovery.
Generally the programs that do the best at bridging these two worlds of disk imaging are the ones at the low end of the spectrum. A program like Acronis True Image or Norton Ghost offers a fast, easy way to recreate your system on another computer. In addition, Ghost, True Image and similar programs can restore individual files and folders from the disk image. Usually these programs sell for $100 or less for single copies (enterprise versions that cover multiple servers are typically more.) Typically these vendors emphasize ease of use and flexibility in the design of their products. Recent versions will automatically export images to a wide variety of media, from USB to tape to DVDs, or over the internet to a remote site.
While all of them handle the basic disk imaging jobs, they tend to differ quite a bit in the other features they offer. For example, Ghost 14.0 offers secure backup to a remote FTP site and some versions of TrueImage have Universal Restore to make it easier to move Windows images to different hardware. However, there are a few things you need to understand before you build your disaster recovery plan around disk imaging.
First, the image's "copy of everything" includes any flaws in the disk image. If you take an image of a system that's been corrupted or infected with a virus, you're going to get a recovered image that's just as corrupted or just as infected. Most of the imaging programs can automatically create an image with user specified options at user-defined intervals. They can also create incremental images between full backups to speed the process and reduce network load.
Generally speaking, you can't simply install an image of a Windows operating system on another computer and expect it to run. Windows uses a structure called the hardware abstraction layer (HAL) which interfaces to the hardware itself. If the hardware configuration on the target system doesn't exactly match the configuration of the original system, the attempted installation will fail with a STOP: 0x0000007B error. To get around this, some imaging programs provide utilities, such as Universal Restore available for Acronis True Image enterprise ("Echo") versions, which allow the image to work on different hardware.
Most importantly, using drive images for disaster recovery does not eliminate the need for thoroughly testing your disaster recovery plan, including the ability to actually install and use the image on your target hardware.
About the author: Rick Cook specializes in writing about issues related to storage and storage management.