Chazz,
With respect, that is a common misconception, but a misconception none the less. Aperture allows you to store your master or original images anywhere you want, inside the Aperture package (which is a form of folder) or on any local drive, even a CD-ROM. And you can mix and match as you wish. In my case, my OS and applications, including my Aperture library, reside on a 500 GB SSD. There is nowhere near enough room on that drive so most of them live on a multi TB HD. In my case, I unload my camera directly into Aperture and park the images inside the Aperture package (called "Managed") to start with as this gives me the greatest possible speed for adjusting big 50MB RAW images. That done, I "relocate" them to the HD, at which point they are referred to as "Referenced" masters in Aperturespeak. My son in law goes one step further, parking his masters on a HD connected to his Airport. Apple does not technically support this, but as his primary machine is a 500GB MacBookPro, he, like me, does not have enough room on his SSD for his enormous panoramas. And, this allows him wireless access to his masters from anywhere in the house.
Bottom line, Aperture masters (and previews and thumbs) are stored in standard, Macintosh files. In fact, each time I download a new release of Lightroom to try it out, I simply have it read the same master image files that I use for Aperture.
As a final note, I would be very surprised if Lightroom stored RAW adjustments in non proprietary files and would love to see documentation of that. Nikon can do it, by writing data back to an extension of the NEF, but I have never heard of any third party being able to do the same. (Doesn't mean that it doesn't happen, but I just don't know how you could do it.)
I know that there is guideline of no more than 10,000 images in a single project, but I have never heard of an Aperture library that was too large to open.