The 3-2-1 rule

For home use, it's just two different devices that can be the same type, like harddrives in two servers, but not redundant data storage in one device, like RAID or just having two copies of files on the same drive. For corporate, most will probably interpret it as two different media types, like harddrive and tape. You want them seperate to prevent accidental deletion of files, ransomware and such.

Diversity in Failure Modes: Different storage media have different failure modes and life expectancies. For example, a hard disk drive (HDD) might be susceptible to mechanical failure, while a solid-state drive (SSD) might have limitations in terms of write cycles. By diversifying the media types, you reduce the risk that a single failure mode (like a power surge, mechanical wear, or temperature sensitivity) could compromise all of your backups.

Reducing Common Points of Failure: If all copies of your data are stored on the same type of device, they may all be vulnerable to the same type of failure. For example, if you have all your backups on different HDDs from the same manufacturer and there's a manufacturing defect, all your backups could fail simultaneously.

Technology and Ageing: Different technologies age and become obsolete at different rates. By using multiple types of media, you're less likely to find yourself in a situation where all your backups are stored on outdated or unsupported technology.

Physical and Environmental Threats: Different types of media have varying levels of resilience to physical and environmental threats like fire, water damage, magnetic interference, etc. By diversifying, you increase the chances that at least one of your backup mediums will survive a catastrophic event.

Data Recovery Options: In the event of a failure, different types of media may offer different data recovery options. Some media might be easier or more cost-effective to recover data from than others.