magnetic disk vs memory device

magnetic disk and memory device both are nouns.

Word	Noun	Adjective	Verb	Adverb
magnetic disk	Yes	No	No	No
memory device	Yes	No	No	No

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

magnetic disk: (computer science) a memory device consisting of a flat disk covered with a magnetic coating on which information is stored
memory device: a device that preserves information for retrieval

Other hypernyms of magnetic disk include storage device.

magnetic disk (noun)	memory device (noun)
(computer science) a memory device consisting of a flat disk covered with a magnetic coating on which information is stored	a device that preserves information for retrieval