What does the RAID disk status "Rebuilding" indicate?

The status "Rebuilding" in a RAID setup specifically indicates that data is being written to a newly added hard drive or to a drive that has replaced a failed one. During this process, the RAID controller is reconstructing the data that was lost or initializing the new drive with the existing data from other drives in the array. This operation is crucial for restoring redundancy and ensuring that the RAID array operates normally and reliably again.

As the RAID rebuilds, it uses parity information or duplicated data from the other drives to reconstruct the lost data. It is a positive indication that the system is actively working to restore its previous state of data protection. It’s important to monitor the rebuilding process, as system performance may be affected, and if another drive fails during this time, it can lead to a complete data loss if there is no other redundancy available.

Other statuses, such as degraded or functioning normally, do not convey the active process of writing data to restore the array's integrity, nor do they indicate that the system is currently recovering from issues.