Have you verified file permissions on the external drive? To avoid permissions problems on an external disk, I just enable "Ignore Ownership" for the entire volume using the Get Info command for it. I do this only for non-boot data only volumes where Aperture libraries are stored. (Actually you can't do it for boot volumes anyway)
You'd think doing the Aperture repair commands would fix this but maybe it only fixes Managed libraries and the referenced files need to be done by the Finder.
Thanks, Thomas. No, we've not verified file permissions on the external drive, but we'll try your "Ignore ownership" suggestion and will report back.
But since Aperture is perfectly capable of relocating the orignal (managed) images to that same external drive, it seems unlikely that it's a permissions problem.