Inability means "lack of ability," "lack of capacity, power, or means": "His inability to pass a driver's test disturbs him." Disability also implies lack of power or ability, but a lack due to some permanent flaw, weakness, or handicap, either mental or physical: "Jeb was excused from service in the armed forces because of a serious physical disability." Inability is traceable to some inherent lack of talent or power that may or may not be due to actual disability: One's inability to speak may have been caused by a birth defect; his disability in speaking may have been caused by an injury.
Dictionary of problem words and expressions. Harry Shaw. 1975.