According to Engadget, the 3-2-1 rule is a long-standing framework for data backup that helps protect against hard drive failure, theft, fire, and other disasters. The rule was popularized by photographer Peter Krogh in his 2009 book The DAM Book: Digital Asset Management for Photographers. It calls for three copies of your data, stored on two different types of media, with one copy kept offsite.
The logic is straightforward: a single copy on one device is vulnerable, and even multiple copies in the same location can be lost in a single event. An offsite copy adds a layer of protection, provided it is located far enough away that a local disaster would not affect it as well.
The article notes that the rule has evolved with technology. Cloud services and automatic backup tools are now common ways to satisfy the offsite requirement, though users should be aware that syncing services may delete files from the cloud if they are removed from a connected device. Engadget also points out that an offsite copy can still be at risk from ransomware if the device is networked, so the rule is best treated as a starting point rather than a complete security strategy.