If the suspension fails to maintain the flying attitude of the head or the smallest bit of dust makes its way onto the disk, the head's flight is disrupted and it may "crash" into the disk, scratching it. Depending on how the drive is being used the data may be lost or may require it to be recovered through a time consuming process. Hard disks are amazingly reliable with some achieving "Mean Time Between Failures" (MTBF) of up to 2,500,000 hours. That means that, on average, a drive fails after the specified number of hours, with half the drives lasting more than that and half lasting less. To put this into perspective, 2,500,000 hours is more than 250 years.