Failed or degraded RAID arrays.

A RAID (Redundant Array of Independent Disks) is a system of combining multiple hard drives into a single unit for enhanced performance, redundancy, or a combination of both. When a RAID array fails or becomes degraded, it means that one or more of the drives in the array are no longer functioning as expected. This can lead to data loss or reduced performance, depending on the RAID level and the specific configuration.

Here are some common scenarios for failed or degraded RAID arrays:

1. Drive Failure: The most common reason for RAID failure is a physical failure of one or more drives in the array. When a drive fails, it can cause the entire array to become degraded or fail, depending on the RAID level.
2. Multiple Drive Failure: In some RAID configurations (like RAID 0), if more than one drive fails simultaneously, the entire array can fail and data can be lost.
3. Incorrect Rebuild: If a drive is replaced, it's critical to ensure that it's rebuilt correctly. If there are any issues during the rebuild process, it can lead to a degraded array.
4. Controller Failure: The RAID controller is responsible for managing the array. If the controller fails, it can result in a failed or degraded array.
5. Power Surge or Spike: A power surge or spike can lead to hardware failure, including the drives in a RAID array.
6. Firmware or Software Issues: Problems with the firmware or software controlling the RAID can lead to a degraded or failed state.
7. Human Error: Accidental deletion of critical data or misconfiguration of the RAID can lead to a degraded state.
8. Overheating: Excessive heat can cause drives to fail, which can lead to RAID degradation.
9. Incompatible Drives: If a replacement drive is not compatible with the existing drives in the array, it can lead to a degraded state.
10. Aging Drives: Over time, drives can degrade naturally due to wear and tear, potentially leading to RAID failure.

When a RAID array fails or becomes degraded, it's important to address the issue promptly. This might involve replacing the failed drive(s), rebuilding the array, and restoring data from backups if necessary. In some cases, it might also be necessary to consult with a professional or the manufacturer for specialized support.