Thanks for giving me a chance to recommend iPhoto Library Manager. This is an AMAZING third-party program that costs $30 and can properly, safely and quickly split or merge iPhoto libraries and do other things as well. VERY well worth the paltry cost, even in these the last days of iPhoto as we know it.
ANY program is going to struggle at least a bit with a 500GB photo library -- I'd suggest you break up your iPhoto library into a set of libraries based on year (or whatever criteria you prefer). Because you're only breaking up a monolithic library you can do this manually, but ILM is much faster and easier.
Thanks for the idea for breaking libraries up with the 3rd party program. Thats a great idea. I came across this post while trying to puzzle my way around another problem. I just bought my wife a new 2014 Macbook Air 250GB/4GB RAM/13" ...yay for me, more cool points for me! The only problem is that while executing the transfer of her "stuff" from her MBP (circa 2009), I found out her current library is 230GB....doh! Short of returning this one and getting the 500GB one, are their any Best Practices that I can do to help reduce her library size? Are their NAS options that iPhoto will recognize, external BT drives....any ideas are appreciated!