Some hard disk drives simply fail because of worn out parts, others fail prematurely. In other words the disk platter (see Illustration) has craters or damage to it. This happens with normal wear and tear. It can be sped up by slamming the lid down on laptops or a tower getting hit by a vacuum cleaner or a foot. Please note the very small space between the read/write head and the disk platter.
Drive manufacturers typically specify a mean time between failures (MTBF) or an annualized failure rate (AFR) which are population statistics that cannot predict the behavior of an individual unit. These are calculated by constantly running samples of the drive for a short amount of time, analyzing the resultant wear and tear upon the physical components of the drive, and extrapolating to provide a reasonable estimate of its lifespan. Hard disk drive failures tend to follow the concept of the bathtub curve.[3] Drives typically fail within a short time if there is a defect present from manufacturing. If a drive proves reliable for a period of a few months after installation, the drive has a significantly greater chance of remaining reliable. Therefore, even if a drive is subjected to several years of heavy daily use, it may not show any notable signs of wear unless closely inspected. On the other hand, a drive can fail at any time in many different situations.