Another alternative might be to run Nikon's Preview creator, found in the Nikon software folder (/Applications/Nikon Software/Nikon View 6/NPrevCreator3, on a Mac). It creates 1600px previews for older Nikons that don't (D1 series) and automatically skips NEFs that have them already. It's part of Nikon View 6 and works fine with NX present. Good luck!