We run into frequent style questions about particular spellings of words and how to use them. You know the kind, something like:
In the clause,
she wanted to backup her hard drive,
should it be backup, back up, or back-up?
The best way to resolve these kinds of issues is by checking the dictionary. I know, who'd'a thunk it? But these are basically spelling and usage questions, and that's what dictionaries are for. Not just definitions, but all kinds of other things, too.
Different dictionaries might resolve these questions in different ways, so the choice of dictionary is no small matter. Many publishing houses, newspapers, and magazines specify which dictionary to use. If you're writing for a particular target and you know which they prefer, use that one. Otherwise, pick one and stick with it. (I've been known to consult multiple dictionaries to try to discover the consensus opinion on various points of usage, but that's mainly because I was charged with the duty to resolve these questions in-house. And because, you know, I'm me.)
So let's take a look at one of my favorite online resources, the Oxford English Dictionary. (Oh, how I love the OED! insert rapturous swoons here.) We'll start with the entry for backup -- one word, no spaces or hyphens:
1 help or support: no police backup could be expected
a person or thing that can be called on if necessary; a reserve:
I've got a security force as backup
the filter is an excellent backup to other systems
[as modifier] :a backup generator
There's more to this entry, but this is enough for our purposes. The first thing we look at is the way the entry is spelled. Does it match what we're looking for? No spaces, no hyphen, one word -- yes, this is what we want.
Next, we look at the part of speech specified in the entry. See those two bolded words? Noun, modifier. This is how this particular spelling can be used. So this should be triggering some warning lights for careful readers right about now. Why? Look at our sample sentence again. How do we use backup in that sentence? As a verb. Not as a noun, and not as a modifier.
So now we know we have to dig deeper and find the entry for backup as a verb. If we try a different spelling variation -- this time, back up with a space, no hyphen, two words -- we find these notes under the entry for back as a verb:
1 (of vehicles) form a line due to congestion: the traffic began to back up
2 (of running water) accumulate behind an obstruction.
back something up
make a spare copy of data or a disk.
(usually be backed up) cause vehicles to form into a queue due to congestion:
the traffic was backed up a couple of miles in each direction
Aha. There it is. Multiple ways to use back with the adverb up, which is exactly what we need. Two words, no hyphens.
You see how that works? This might seem like very basic information if you already know how to use a dictionary, but we get so many usage questions that I thought it might not be a bad idea to go over the method for checking these things.