If a drive fails in a RAID 1 array, the system remains operational as the data is still accessible from the mirror drive(s). It is important to replace the failed drive as soon as possible to restore full redundancy and ensure data protection. Once the failed drive is replaced, the RAID controller or software will rebuild the mirror by duplicating the data onto the new drive. During the rebuild process, the array operates in a vulnerable state, as another drive failure could result in data loss. Therefore, monitoring drive health and performing regular backups are essential practices to maintain data integrity in a RAID 1 array.