hard disk vs magnetic disc

hard disk and magnetic disc both are nouns.

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

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

hard disk: a rigid magnetic disk mounted permanently in a drive unit
magnetic disc: (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 disk.

hard disk (noun)	magnetic disc (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