It depends what type of drive and array you are using. Most modern drives have SMART (Self-Monitor Analysis and Reporting Technology) and there are at least 3 versions of SMART. In its original incarnation SMART provided failure prediction by monitoring certain online hard drive activities. A subsequent version improved failure prediction by adding an automatic off-line read scan to monitor additional operations. The latest SMART technology not only monitors hard drive activities but adds failure prevention by attempting to detect and repair sector errors. Also, while earlier versions of the technology only monitored hard drive activity for data that was retrieved by the operating system, this latest SMART tests all data and all sectors of a drive by using off-line data collection to confirm the drive’s health during periods of inactivity. There are a number of different tools that can be used to obtain the status from each HD from SMART. But this generally requires direct access, as opposed to logical access which is what happens in a RAID array. With RAID arrays they will generally provide vendor specific software that can monitor and read the SMART information from each HD to predict failure.