Hard drive reliability is closely related to temperature. By operational design, the ambient temperature is 86°F. Temperatures above 122°F or below 41°F, decrease reliability. Directed airflow up to 150 linear feet/min. is recommended for high speed drives.
Mean time between failure (MTBF) and Annual Failure Rate (AFR) is based on design and historical data (i.e. Bellcore). This is a predicted theoretical MTBF. The failure rate does not include drive returns with no trouble found, excessive shock failure, or handling damage. Historically the field MTBF, which includes all returns regardless of cause, is typically 50-60% of projected MTBF. A standardization effort is underway in line with IDEMA Assoc. AFR/MTBF methodology, and pending development of an operational MTBF and derating curve.