back up vs affirm

back up and affirm both are verbs.

Word	Noun	Adjective	Verb	Adverb
back up	No	No	Yes	No
affirm	No	No	Yes	No

As verbs, affirm is a hypernym of back up; that is, affirm is a word with a broader meaning than back up:

Other hypernyms of back up include confirm, corroborate, substantiate, support, sustain.

back up (verb)	affirm (verb)
establish as valid or genuine	establish or strengthen as with new evidence or facts
become or cause to become obstructed	to declare or affirm solemnly and formally as true
make a copy of (a computer file) especially for storage in another place as a security copy	say yes to
move backwards from a certain position
give moral or psychological support, aid, or courage to