I'm doing something for school on this topic and I'm reading a manual from the International Association of Sound and Audiovisual archives and at one point it says:
While experts tend to agree that the most reliable data system consists of a HDD array supported by multiple duplicates on tape, the continued reduction in costs and improvement in reliability make the concept of identical duplicates of data on separate hard disks a possibility. The principle of multiple media remains, however, and disk only storage constitutes a risk.
This "save the data on different technologies" theme is repeated a few times throughout, but I don't understand why disk only storage would be a risk.
If you have a copy of a file on three disks, with each disk at a different location, why would this be considered risky? What are the odds that all three of them are going to drop dead at the same time?