Which of the following is a reliable way to test backed up data?

The most reliable and comprehensive method to test backed-up data is to perform a full restoration to a separate, non-production system. This procedure validates the entire recovery chain: the integrity of the backup media, the functionality of the backup and restore software, and the usability of the recovered data itself. Successfully restoring and verifying the data provides the highest level of assurance that the backup is viable and can be depended upon in a real disaster recovery scenario. While other methods provide partial checks, only a full restore tests the end-to-end process.

A. Verifying the file size is unreliable; it does not confirm data integrity, as a corrupted file can still have the correct size.
B. Confirming the backup service is running only verifies that the backup job was initiated, not that the data was successfully and correctly written.
C. Comparing data hashes is a strong method for verifying data integrity but does not test the restorability of the media or the recovery process itself.

1. National Institute of Standards and Technology (NIST) Special Publication 800-34 Rev. 1, Contingency Planning Guide for Federal Information Systems. Section 5.3, "Plan Testing, Training, and Exercises," emphasizes that "Recovery strategies should be tested to validate that they will work as planned." Table 5-2, "Contingency Plan Test, Training, and Exercise Activities," explicitly lists "Restoration of a single system from backup media" as a key testing activity.

2. MIT Computer Science and Artificial Intelligence Laboratory (CSAIL), The Missing Semester of Your CS Education. In the lecture on "Backups," the courseware states, "How do you know your backups work? You have to test them. Try to restore them and see if you get the data you expect." This highlights restoration as the definitive test of a backup's viability.

3. Microsoft, Azure Backup documentation. In the "Best practices for Azure VM backup" guide, Microsoft recommends, "Perform periodic restore drills... Testing the restore process provides confidence in your DR [Disaster Recovery] strategy and validates that your backups are recoverable." This vendor best practice underscores the importance of testing the full restore process.