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Image Backup
This is the only form of backup that allows you to completely recreate the software environment that allows you to replace the hard drive and quickly get your computer back to running like it did with the old hard drive. It backs up the Operating System along with everything else. This requires special backup software, like Norton Ghost or Acronis True Image.
If you have a DCI computer, your DCI System Restore Discs were created with backup imaging software. It is an image backup of your computer as it was when it was originally purchased.
Full Backup
This backs up almost all of the files on the hard drive. Files that are open while you run the backup will often fail to be saved in a useful form. It will not successfully back up the Operating System or the functional settings for the applications. It does, however, back up all the files you can think of and more, so having a full backup prevents you from wishing you had remembered to back up a file that had been saved in a folder you forgot to back up.
You can get full backups in any normal backup program like the backup program built into every version of MS Windows or in Retrospect or other commercial backup software or the software typically bundled with most external hard drives.
Selective Backup
In order to back up the contents of a large hard drive onto a medium that is too small to back up everything, you must choose to back up some of your files and leave the rest at risk. You can do this using any of the same backup programs that create full backups.
Many people choose to back up their pictures, documents, and other unique files and expect to reinstall the Operating System and applications if the hard drive crashes or if they are hit by a bad virus. If you just back up your My Documents folder, you'll get most of these files.
If you back up to the U.Va. Home Directory Service or a USB FLASH drive or other small capacity medium, you need to consciously decide what files are most important to you. If you collect digital photographs and music, you might not be able to fit everything in My Documents on such backup media. The question then becomes how you select which files to back up.
There are also more automated ways of selecting files to back up. Differential and incremental backups are basically automated selective backups. Begin with a full backup. After some files change, you can use differential and incremental backups to back up only the files that have changed since your full backup.
Incremental Backup
If you have already created an image backup or a full backup, your operating system has marked those files as backed up. Later, you save new files to your hard drive or modify old ones. These changes are not in your backup. Norton Ghost and any of the normal backup programs also offer the option of automatically backing up only the files that have changed since your most recent backup. Once this incremental backup is complete, these new and changed files are marked as backed up so that when you do another incremental backup, these files will not be backed up again.
Each incremental backup is relatively small and quick to make, but if you later need to fully restore your hard drive, you will need to restore the most recent full or image backup and then follow that by restoring each of your incremental backups in the order they were recorded. You can later shorten this list of backups you need to restore by making a new image or full backup.
Differential Backup
The differential backup works exactly like the incremental backup, except that when the smaller backup is done, the new and changed files are not marked as backed up. This means that when you do subsequent backups you will back up all the files that are not backed up in the most recent full backup.
Pros and cons of differential and incremental backups
Incremental backups are more thorough. You can restore files that were deleted or overwritten after your last full backup, but before your most recent incremental backup. Finding a specific file amid a collection of incremental backups can be challenging, though. Many backup programs offer indexes that help you find specific files.
Differential backups offer simpler full restores. Just restore your most recent full backup and your most recent differential backup and you are done. You will lose files, however, if they were created after your most recent full backup and deleted or overwritten before your most recent differential backup.