How big is your system drive? It is recommended that you leave 15-20% of your system drive free -- particularly if you use Aperture.
With only 15 GB free, it sounds as though you
- need to free up some space or buy a bigger drive, and
- are better off leaving your Library on your external drive
I haven't experienced any noticable slowdown with Libraries on FW800 drives as compared to the same Libraries on my system drive. On my system (MPB 5,5 w. 8 GB RAM and a 500 GB HD) the bottleneck seems to be CPU, GPU, and RAM, not HD throughput.
I'm not saying a Library on a system drive isn't faster -- just that it's not a lot faster, may not be noticeable -- and in your case may not be possible.
I certainly recommend against using the system drive for a Library if it means leaving less than 15% of that drive empty. Afaik, most posters here agree.
What is your goal here? Your thread has wandered over a lot of ground, but I'm still not sure what you want to do with Aperture, or why.
(Sent from my magic glass.)
I realize this thread is over a year old, but I had the exact same question as the original poster and had high hopes that it would help me achieve the same thing--take an aperture library that was already on an existing external hard drive and move everything but the masters over to an internal drive. Since it didn't seem to get answered, I thought I'd at least provide an answer in case anyone else had the same question and ended up here.
Turns out it was a very simple process. All I had to do was create another folder on the external drive (like "Aperture Masters") and then go into the Aperture library running on that external drive and go through the process of relocating the originals to that new folder (if you need help on that, there are a ton of threads on how to do it).
At this point, Aperture will take all of the originals out of the managed library and put them in that new folder. When it's complete, just copy the Aperture library to your internal drive and when you open it, it should automatically connect to the files in that new folder that you made. Far easier than I had thought it was going to be and Aperture really makes it painless.
Just back up your library before you do it!