What method involves copying only the data that has changed since the last backup?

The method that copies only the data that has changed since the last backup is known as an incremental backup. This approach is efficient because it minimizes the amount of data that needs to be backed up each time, which saves both time and storage space. After a full backup is performed, the incremental backup captures only the changes made since the last backup of any type, whether it be a full backup or another incremental backup. This means that subsequent backups will only include new or modified files, making it a highly efficient option for ongoing data protection.

In contrast, a full backup captures all data every time it runs, which requires more time and storage capacity. Differential backups, while they also track changes, capture all the data that has changed since the last full backup, leading to larger backup sizes over time as more changes accumulate. A mirror backup creates an exact copy of the source data, but it does not retain historical versions of files that have changed or been deleted. This differentiation helps to highlight why incremental backup is the preferred method for efficiently managing storage and backup frequency.