Ok so I asked a few people, and most of them agree that backing up would work.

I'm on a laptop until tomorrow, when I assemble my computer. I'd like to be able to play a game on it though, when it is ready. I recently bought Just Cause 2. Can I download the game on my laptop, and then back it up to my external, so that when I have the new computer, Steam can just copy the files over and install the updates?

If you back up your Steam/steamapps/common folder somewhere, you can put that in any other Steam install you might have and Steam will simply have to download a few small files to get it ready to go. And within that common folder are the individual game folders, so you could pick and choose if you want to. Just make sure that you keep the subfolder structure right. :)

Yeah the steamapps suggestion is the easiest one, just drag & drop the folder into your new install of steam either over the network or via external storage and you're good to go (as a tip, be sure to install steam on your biggest HDD because the size of that folder shoots up quick).

Its easily the best thing about steam, when I set my new PC up I had 20 odd games that average 2-3gb each installed in about 5 minutes

whoops I wish I could mark two answers lol.

Thanks, i'll download it tonight and copy it to my external!

Jerk! :)

No problem. Glad to be of help.

Good to see you figured this out!

The built in backup is still really flawed, meaning it will have to download a whole load of content again after you're done. So manually backing up is a much better option.