I just have a little to add -- I haven't had time to investigate. Starting c. six weeks ago, every time I relocate Originals from the Library to an external drive, Aperture recreates all the Previews. I don't know why. I have never changed my Previews setting, and I always have Aperture create Previews for all Images.
Some things to check:
- does it depend on the file format?
- does it happen if you now consolidate some Originals, and then relocate them to an external drive again?
- does it add more Previews, or is it replacing the existing Previews?
- does it happen with "Share Previews" off?
I can confirm that it doesn't matter whether the file has any adjustments or not.