Our interpretation of uncorrectable bit error rates is that they represent the rate at which errors are detected during reads from the disk during the normal operation of the disk drive . One approach that can be used with or without redundancy is to try to protect against bit errors by predicting when a disk is about to fail. VAXsimPLUS, a product from DEC, monitors the warnings issued by disks and notifies an operator when it feels the disk is about to fail. For example, an accident might sharply increase the failure rate for all disks in a disk array for a short period of time. In general, power surges, power failures and simply switching the disks on and off can place stress on the electrical components of all affected disks.
Disks are generally more likely to fail either very early or very late in their lifetimes.
Early failures are frequently caused by transient defects which may not have been detected during the manufacturer's burn-in process. Late failures occur when a disk wears out. Correlated disk failures greatly reduce the reliability of disk arrays by making it much more likely that an initial disk failure will be closely followed by additional disk failures before the failed disk can be reconstructed. The above three failure modes are the hardest failure combinations, in that we, currently, don't have any techniques to protect against them without sacrificing performance.