A general heuristic is: single word is the noun form (e.g. "I made a backup"), two words is the verb form (e.g. "back up your data"), and hyphenated words is th

A general heuristic is: single word is the noun form (e.g. "I made a backup"), two words is the verb form (e.g. "back up your data"), and hyphenated words is the adjectival form (e.g. "the back-up copy is over there").