Partial is an adjective with two distinct meanings. The first meaning is "biased," "prejudiced," "showing favoritism," in such a phrase as "to be partial to." The other meaning of partial is "not complete or total"; in this meaning the adverb partially is a synonym of partly. Usually, you should prefer partly to partially unless the meaning is that of limited degree."Jeb has always been partial to blue-eyed girls." "This is a partial payment of what I owe you." "The accident partially blinded him." "His partial knowledge of the subject was due partly to his youth."
Dictionary of problem words and expressions. Harry Shaw. 1975.