To stop Photos opening when you connect a camera...
Open Image Capture (Applications > Image Capture)
Connect your D810 by USB.
Once Image Capture has recognised your camera go to the bottom of Image Capture's left-hand column and click on the little up arrow...
It will have your camera and "Connecting this camera opens:'
Select Image Capture as the opening program. This will stop Photos from opening whenever you connect that camera.
This seems to be the only way you can do this.
(Apple seem to be making it ever more obscure to find your choices and control your own devices (computer/camera etc).
It's a great shame as less and less control is left to the user. Image Capture has been around ever since OS X and it's the only program on your Mac that lets you save your photos (AND videos!) where you want to save them - i.e. in a folder that you know where it is.)
You can still bring those photos into Photos if you want to (File > Import...) but before you do, make sure you go into Photos > Preferences... and UNcheck 'Importing: Copy items to the Photos library'.
This prevents the Mac duplicating all your photos into the Photos library.
Having your photos stored outside of Photos' own library does not affect Photos' performance.
Hope that helps.
