back up vs sustain

back up and sustain both are verbs.

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

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

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

back up (verb)	sustain (verb)
establish as valid or genuine	undergo (as of injuries and illnesses)
become or cause to become obstructed	establish or strengthen as with new evidence or facts
make a copy of (a computer file) especially for storage in another place as a security copy	admit as valid
move backwards from a certain position	supply with necessities and support
give moral or psychological support, aid, or courage to	provide with nourishment
	be the physical support of; carry the weight of
	lengthen or extend in duration or space