Why is a failed disk replaced by a hot spare?

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When a disk fails, the goal is to restore redundancy and keep the system online as quickly as possible. A hot spare is a disk that is already connected and ready to use, so the controller can immediately start rebuilding the missing data onto that spare. This minimizes the window during which the array is degraded and lowers the risk of another failure causing data loss, helping maintain continuous availability and system performance during the rebuild. The other options aren’t about restoring availability or speeding up the rebuild process: hot spares aren’t primarily for saving power, testing degraded performance, or moving data to cold storage.