Which is correct?
- To back up data.
- To back data up.
The context is the following:
He was careful enough to perform tests and [back up data | back data up] to avoid any problems.
Back up data is by far the most common, (referencing two Google searches (1780000 results vs. 18200) and Ngrams), and sounds the most natural. This is because back up, though written as two words in verb form, is spoken (and thought of) as one word. To split the “verb” up and put something in between (e.g. data) would be to confuse the meaning of the sentence for most people.
He was careful enough to perform tests and back up data to avoid any problems.
Backup as one word is a noun/adjective, and is not applicable as a verb.
The backup software on this computer is ancient.
The backup runs from 12 AM to 1 AM every night.
Edited to say that I wholeheartedly agree with @tdhsmith’s comment:
…while keeping the “verb” whole is generally more readable with wordier objects, most native speakers must split it when using a pronoun. For example, “I back it up every night” is correct to me but “I back up it every night” feels very wrong. [See below for the entire comment]