[Trellmor]

I don't know where you want to backup you games, but you can copy just the one game folder and remove the game afterwards from steam. To restore it move the folder back and "install" the game. Steam will verify the files and only download new/changed files. Should be faster the the backup option inside steam itself.

If you have a second hard drive (or maybe network drive, haven't tried) you can also create a steam library folder on that drive and just move the game to that library from within steam. That way it will still get updated.