>I do understand that, but what are the issues with having the catalog on one drive/location and the image files on another drive?
None. The implementation of this is not so slick as it was in Aperture but it works just fine. I keep terabytes of images on separate hard drives and they are all referenced in my main Lightroom catalog. To edit them in Develop, all I have to do is hook up the correct hard drive but you can do everything else such as keywording, slideshows, etc. just from the previews in the catalog. It works the same for NAS. Even if you are not near your NAS network, Lightroom will just work fine and show you the images but you won't be able to use Develop. Keep your catalog on your local hard drive and put the images on your NAS. That will work just fine and will make Lightroom feel about as fast as having all images on your local hard drive. You'll just have to relink the files after you move them inside your Lightroom catalog. (see here for how that works: Re-linking Files and Folders in Lightroom « Julieanne Kost's Blog and thousands of other tutorials on this on the web if you google for it). You can also move them inside Lightroom but when you're dealing with this many images I would not attempt that and use the finder to do that.
