SoftRAID uses SMART data from each disk to determine if a disk is more likely to fail. It interprets this SMART data based on the results of a study performed by Google on consumer-grade disk drives. It checks the number of reallocated sectors and the number of unreliable sectors that each disk reports. If either number is greater than 0, the disk is much more likely to fail.
Once SoftRAID predicts that a disk is likely to fail, it may continue to function normally for a while with no outward signs that anything is wrong. In our testing, we have seen disks completely stop working in as little as 24 hours after they were first predicted to fail. Other disks continue to function for a month or two.