fixed storage vs CD-ROM

fixed storage and CD-ROM both are nouns.

Word	Noun	Adjective	Verb	Adverb
fixed storage	Yes	No	No	No
CD-ROM	Yes	No	No	No

As nouns, CD-ROM is a hyponym of fixed storage; that is, CD-ROM is a word with a more specific, narrower meaning than fixed storage:

fixed storage: (computer science) memory whose contents can be accessed and read but cannot be changed
CD-ROM: a compact disk that is used with a computer (rather than with an audio system); a large amount of digital information can be stored and accessed but it cannot be altered by the user

Other hyponyms of fixed storage include compact disc read-only memory, EPROM, erasable programmable read-only memory.

fixed storage (noun)	CD-ROM (noun)
(computer science) memory whose contents can be accessed and read but cannot be changed	a compact disk that is used with a computer (rather than with an audio system); a large amount of digital information can be stored and accessed but it cannot be altered by the user