Have you also checked, if the external drive is formatted correctly? MacOS X Extended (Journaled)? If not, make sure it has the same file system as your internal drive and reformat the external drive, if necessary. Set the "Ignore Ownership on this volume" flag, if it is not set.
This I don't understand:
Thanks for the suggestion but still get the same message after individually copying all subfolders as you instructed in my 2012 library to verify individually.
So you could copy all folders individually; then there is no need to copy the complete library again.
The idea is to copy all folders from your library individually to another folder on your external drive, in exactly the same folder structure as the original library.
Then this folder will contain your transferred Aperture library. The last step would be to rename the enclosing folder "ApertureLibrary2012.aplibrary" and double click it to open it in Aperture and test, if it is working.
-- Léonie