What backup method involves creating a constant backup whenever changes occur?

The method that involves creating a constant backup whenever changes occur is disk mirroring. Disk mirroring is a real-time backup technique where data is simultaneously written to two or more disk drives. This ensures that any changes made to the primary drive are immediately replicated on the secondary drive, providing an up-to-date copy of the data at all times. This approach is particularly useful for disaster recovery, as it minimizes data loss and ensures high availability of data.

In contrast, the other backup methods—incremental backups, full backups, and differential backups—do not provide continuous updates. Incremental backups only back up data that has changed since the last backup, requiring a sequence of backups to restore fully. Full backups create a complete copy of all data at a specific point in time, but they are not performed continuously. Differential backups take all changes made since the last full backup, which also means that they are not constantly updated in real-time. Thus, disk mirroring stands out as the method that provides real-time duplication of data changes.