sustain vs back up

sustain and back up both are verbs.

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

As verbs, back up is a hyponym of sustain; that is, back up is a word with a more specific, narrower meaning than sustain:

Other hyponyms of sustain include back, vouch, verify, demonstrate, establish, prove, shew, show, document, validate.

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