The 3-2-1 rule:

  • 3 – Keep 3 copies of any important file: 1 primary and 2 backups.
  • 2 – Keep the files on 2 different media types to protect against different types of hazards.
  • 1 – Store 1 copy offsite (e.g., outside your home or business facility).

Read more: Data Backup Options from US-CERT (United States Computer Emergency Readiness Team)