The Mechanism
Enterprise-grade hard drives feature a technology called TLER (Time-Limited Error Recovery). When an enterprise drive hits a bad sector, it spends only a few seconds trying to fix it before reporting the error to the controller, allowing the controller to handle it using RAID parity.
The Impact on RAID 5
If consumer-grade drives (like standard desktop HDDs) are used in a RAID 5 array, they do not have TLER. A consumer drive may freeze for 30 to 60 seconds trying to recover a bad sector. The RAID controller, expecting a fast response, assumes the drive has completely died and forcibly drops it from the array. If two consumer drives hit bad sectors around the same time, the controller will drop both, instantly killing the RAID 5 array even though the drives are physically intact.
