RAID
An array of drives configured for redundancy, speed or both, presented to the system as a single volume.
In depth
RAID protects against drive failure, which is a genuine risk, but it does not protect against deletion, corruption, ransomware or theft — every one of which propagates instantly across the array. It is an availability measure, not a backup, and treating it as one is among the most common causes of catastrophic archive loss.
Examples in practice
A file deleted by accident vanishes from every disk in a mirrored array at the same instant.