Aperture regards masters as previews

Hi,

I recently moved from iPhoto to Aperture. I opened my existing iPhoto library, which holds 25k pictures, in Aperture (so I did not use the File -> Import option, I simply opened the existing iPhote library).

Out of the 25k photos, roughly 500 of them have a badge with showing a yellow warning sign (I filtered on "File Status" is "Missing").

After some investigation through the Finder, it turns out that for these 500 photos, Aperture is missing the master files. So there is no photo in the My iPhoto Library / Masters but there is a photo in My iPhoto Library / Previews. However, these photos are actually masters, not previews, judging from the pixel size (3008 x 2000, which is the same as the other photos I took with my Nikon D70 at that time).

It looks to me that somehow Aperture regards these photos not as masters, but as previews. I can't find a pattern which of the 500 images are affected by this, it seems to have happened to some random pictures in my library.

Is there an easy way to let Aperture recognise these photos as masters again? I guess one option could be to copy each image to e.g. the desktop and import them again. How do I ensure Aperture links these newly imported photos to the photos regarded as previews?

Would the problem be resolved if I _import_ my iPhoto library into Aperture rather than just open it? What are the differences/benefits of importing an iPhoto library compared to opening an library in iPhoto?

Thanks,

Ivo