Bondiblue,
What previous library did you try to convert? A Lightroom Library? You know you don't have to use the internal database managed library. You have been able since Apertur 1.5 to use a "referenced" library, just like LR. When using a referenced library, it takes much less time to import the images since it doesn't have to copy them to a new location. But it locks you out of using the Vault. And it has the same problem that a referenced library always has. If you fool with the images outside of the program, or move or delete them, it messes up the internal database of both LR and AP.
First, when trying a new program, it is never a good idea to do wholesale importing. Import a few images into a single Project until you figure out what you are doing and how the program works. Then read up in the online manual about how the library works and best practices for importing images. I really believe that a lot of the importing problems folks are experiencing are because they just jump in with both feet without knowing anything about how image importing and cataloguing in Aperture works.
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Only my opinion. It's worth what you paid for it. Your mileage may vary! ;-}
http://www.dougwigton.com/