Since upgrading to Aperture 3, the size of my vaults has exploded to more than 60GB, even though all my masters are referenced (residing outside the aperture library). This means that the vaults don't fit on the external hard drives anymore.
The message "Copying Previews" during the vault update process would suggest that the previews (I have plenty of those) are included in the vault. I had thought that the previews were NOT included in the vault, but just the information required to recreate them, if need be. (That's what Aperture 2 did, I think)
Can I stop previews from being backed up in the vault?