Understanding Disk Images

What is a "disk image" and how can it give you a truly complete backup? Find out today in Doc's regular Tell Me About column.
Published: Nov 17, 2005
Author: Michael E, Callahan
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This question submitted by Janet Diamond, Alan Finkel, Donna Polaski, John Flannigan, Stephen Stampau and numerous others

As anyone who reads my reviews knows, I'm a big advocate of keeping your hard disk backed up. I had one hard disk crash over 20 years ago and since that time I've always kept good backups. In the early days, like the mid-to-late 1980's, you backed up to 5.25" floppy disks that held 360K. I know, hard to believe. In the early 1990's you backed up to 3.5" floppy disks, and you could also backup to tapes. And today, with DVD drives so prevalent, many people backup their computers to CD or DVD. Yet another option is the disk image which always leads to the question, "What is a disk image?" I'll tell you.

When you do most backups you are copying the contents file by file, folder by folder. Even on a fast computer this can be very time-consuming. A disk image, however, is a file that contains everything on your hard disk. It's like a picture, and sometimes you'll hear an image called a "snapshot." An image is an exact copy of not only the contents of your device, but also the actual structure of the device. When you backup individual files, that's what you have - a bunch of files. When you create an "image" of your disk, however, you effectively backup the operating system, the registry and every single file.

Now, if you think about it, an image is going to be quite large. The image of my drive C, for example, is 11 gigabytes. For storing my disk images I have an external Maxtor "One Touch" hard drive that holds 150 gigabytes. You could also save images to CD or DVD. The standard DVD holds 4.7GB so it can hold a good-sized image. Some software programs can also compress the disk images so it takes up less space.

One of the best-known disk images types is the one for CD-ROMs, which utilizes the ISO 9660 file system. An image of a CD has the file extension .ISO. If you've downloaded a version of Linux, for example, you most likely downloaded an .ISO file which you then "burn" to a CD. The benefit of doing backups with a disk image is, to me, substantial. Lets say your hard drive crashes. With backups you can replace your data, but you still have to get a new hard drive, reinstall Windows, reinstall your programs and then, finally, use your backups to reinstall your data.

With a disk image backup, however, you could get the new hard disk and have the image software restore the image of the drive to the new disk. The result? Your hard disk would be back to the exact state it was when the image was created. Complete with operating system, programs, registry, files and data, and you'd be ready to go. Something to think about. External drives are a good option and right now they're fairly inexpensive, costing about $1.00 US per gigabyte. So, you can get a 200 gigabyte drive for you backup images for around $200.00 US.

Here are a few programs here on Tucows that create disk images that you can use for backups.

Products like Norton Ghost are also excellent for creating images of your disk and restoring them. So, if you're looking for a total backup solution you may well want to look into backup software that creates disk images because an image stores everything!

I'd like to thank Janet Diamond, Alan Finkel, Donna Polaski, John Flannigan, Stephen Stampau and numerous others for asking thisquestion.

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About Michael E, Callahan

Michael E. Callahan, known around the world by the trademarked name Dr. File Finder, is regarded as the world's leading expert on shareware. Dr. File Finder works with software programs and developers full-time, and in the average year he evaluates 10,000 programs. Since 1982 he has evaluated over 250,000 software and hardware products. Mr. Callahan began evaluating software online in 1982 and no one has been at it longer. He currently works doing online PR and marketing for software companies, and is the Senior Content Producer for Butterscotch.Com.

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