Today I received the following email (screenshot below) from Nikon Customer Support regarding the issue with the black frame around the image when capturing video on an external recording device (such as the Atomos Ninja or Blackmagic Hyperdeck Shuttle) via the HDMI port of the D600. It looks like they’re escalating this to the engineers (I assume) in Japan. (see the previous post covering the D600 HDMI bug here)
I’ll update with a new post once I hear back from Nikon.
In the meantime, if you are using the D600 with an HDMI recorder, note that you can size out the black frame completely by scaling your clips up to 105.5% (or larger) in your post-production apps (e.g. Final Cut Pro), or via a transcoding tool such as MPEG Streamclip (under the Zoom option). 105.5% is the minimum scale amount I’ve found to work, as 105% scaling as others have suggested still leaves a slight black edge for me. Hope that helps!