6.3.15 Reliability  Loss of data as a consequence of disk failure and head crashes has made most data professionals suspicious of HDDs, however manufacturers now claim annualised failure rates of less than one percent and an operational life of 40,000 hours (Plend 2003). High reliability drives may have an even longer operational life, termed by manufacturers as “mean time between failure”. Though HDDs are self-contained and sealed and so protected from damage, most failures in disk drives occur in two opposing ways: as a result of wear through extended use, or as power to the drive is turned on or off. The dilemma is whether to leave the disk on, and increase wear, or turn it on and off and increase risk of failure.