When Disaster Strikes

One of the 4 disks in the array has failed (sdb in this example). RAID is running on 3 disks, and all data is still there, but there is no more redundancy. If we had a spare, it would have been added to the array and  reconstruction would start in background. After it is done, the array can recover from another disk failure. If autodetection is enabled, the failure is detected and recovered from during boot.