Both image and file/folder backups can be performed in one of the following ways:
- Full backup
- Incremental backup
- Differential backup
A backup program that performs only full backups must back up everything every time it runs. This is the way that the free image backup programs provided by Western Digital and Seagate work. Over time, a full backup-only strategy wastes an enormous amount of disk space.
An incremental backup, by comparison, backs up only the files that have changed since the last backup of any kind, while a differential backup backs up only the files that have been changed or created since the last full backup.
Most commercial backup programs use the incremental backup method for backups after performing a full backup. This is also true of the integrated image+file backup incorporated into Windows 7.
Backup programs and services that provide so-called “continuous data protection” monitor changes to files and back up changed files immediately or within short increments, such as every hour. Check your backup program or service for details.