… is a necessary rule of thumb for computer-based knowledge & design workers. But add the lesson of cloud computing:
Backups: If you don't have 3 copies, you aren't serious.
The standard redundancy for cheap cloud storage options is 3 copies. Anything less is reduced redundancy, sold at discount. You should have at least 2 backups, for instance both a home backup disk and a cloud drive or repo.
A big win, when you plan for multiple copies, is that you no longer need any of them to be highly reliable. What matters more is, how fast can you make another copy if one copy goes down?