hard disk vs magnetic disk

hard disk and magnetic disk both are nouns.

Word	Noun	Adjective	Verb	Adverb
hard disk	Yes	No	No	No
magnetic disk	Yes	No	No	No

As nouns, magnetic disk is a hypernym of hard disk; that is, magnetic disk is a word with a broader meaning than hard disk:

hard disk: a rigid magnetic disk mounted permanently in a drive unit
magnetic disk: (computer science) a memory device consisting of a flat disk covered with a magnetic coating on which information is stored

Other hypernyms of hard disk include disc, disk, magnetic disc.

hard disk (noun)	magnetic disk (noun)
a rigid magnetic disk mounted permanently in a drive unit	(computer science) a memory device consisting of a flat disk covered with a magnetic coating on which information is stored