What is: 3-2-1 backup rule

The 3-2-1 rule says: keep 3 copies of your data, on 2 different media, with 1 copy off-site (ideally offline or immutable).

It is the simplest way to never lose data: if one medium fails or ransomware hits one location, you always have a clean copy elsewhere.

The critical part is the "1 off-site and isolated", together with regular restore testing.