Your RAID arrays that are created from drives consist of drives that are active members and drives that are spares.
The spare drives are used automatically if a member drive fails. If you have sufficient spare drives, you do not have to replace them immediately when they fail. However, monitoring the number, size, and technology of your spare drives, ensures that you have sufficient drives for your requirements. Ensure that there are sufficient spare drives available so that your RAID arrays are always online.