You can use the
rsync command line tool to do a one-way synchronization of your files. The way
rsync works makes it easy to interrupt the process if you don't have the patience for it—you can easily resume it later on. Only the files that are still to be transferred will be copied.
You need to open Terminal.app, and then call
rsync like so:
rsync -avh --progress "~/Pictures/Aperture Library" "/Volumes/NAS/"
Here, the first path is pointing to your library, which by default should be under Pictures. If you're unsure, you can drag and drop the library to the terminal command line and it will fill the path automatically for you. The same goes for the path of your NAS.
-a option enables the archive mode, which sets some defaults, including recursive copying.
-v will make the command more verbose.
-h turns on human-readable file sizes.
Rsync will show you a progress meter. If you want to cancel the process, press CtrlC. You can call the command again to have