Gatkeeper prevents Photos from importing Aperture Libary

I am trying to open a 160GB Aperture Library with the Photos.app The library can be opend with aperture w/o problems. As soon as I start opening it with Photos, Gatekeeper starts checking the files. This seems to take very long. I did let it running for a couple of hours and quit everything.

I tried to disable Gatekeeper with

sudo spctl --master-disable

But that didn't change a thing. It still opens the dialog and start checking the files, with no progress:

Solution 1:

The library was on an external diksdrive. I set the "Ignore Permissions" option with Command-I for that drive. Restartet the Mac and opened the library with Photos and the import was done in several minutes.