Copying the Library using Finder works, but, as you seem to indicate you know, there is no way for Finder to copy only the changed parts within the Aperture Library, and so you have to wait while Finder copies all of the data, even though most of it may be identical to what is already on the back-up drive. What you are asking to do is to make a differential backup (good primer here, though the product is for PCs). Time Machine, OS X's built-in backup application, does that. Many users use Time Machine for one back-up, and purchase other software that clones external drives (or Folders) for other back-ups. (For external drives the need is even greater.) The two commonly used and widely recommended products for Macs are SuperDuper! and Carbon Copy Cloner.
I use SuperDuper. For each Library on an external drive, I have two additional external drives of the same (or greater) capacity. Every day I work on a Library, I clone the drive containing the Library to one of the back-up drives. I keep one of these off-site, and never have all three copies of the Library (the working copy and the two back-up copies) in the same location.
SuperDuper uses something it calls "Smart Update" (iirc). This does exactly what you want: it copies (or changes) only the files that have changed since the last back-up. In practice it is fast. (I am sure CCC has the same function.)