Mac or windows? What version?
What exact NUMBER version of Lightroom? (See the help>system info dialog)
What is the exact wording of the error message?
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Now, about 6 weeks later, I am attempting to import some NEFS from the D810 and it says that it is an unrecognized format. I did nothing at all to this machine, so I assume that an auto update broke something regarding the NEF reading. I mean seriously!???
I don't think so. Please tell about your OS and version and LR version.
Post one of your NEFs to dropbox, because I suppose that those are the problem. Or you have changed something to your system and you are not aware of.
Thanks for looking into this.
The exact error message is ...
The file is not recognized by the raw format support in Lightroom.
Lightroom version: 4.4 
Operating system: Windows 7 Home Premium Edition
This is one of my NEF files that I cannot read with the above Lightroom.
The D810 requires Lightroom 5.6 and you are using Lr 4.4
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You have given the answer to your trouble: the 810 is not supported with your version of LR wich gives you now 2 choices:
- Upgrade to LR 6 or LR CC (do not forget to apply the latest sub version)
- Convert your NEF files to DNG using the free Adobe DNG converter and working with DNGs.
Besides that: The 810 did never work out. May be you worked withz JPEGs or TIFFs or DNGs, but not with your original NEF files or you had a higher LR version number at your disposal.
My last guess is that you had some other camera before and just acquired or shot with a D810.
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If you were previously importing the D810 NEF files, then you have two copies of LR installed, an old and newer version. This happens when you upgrade major versions. With the two copies installed, you would seem to be running the older version. This can easily happen when the install of the newer version does not correctly update the desktop icons. (Happens more often than it should.)
That is exactly what happened. I actually have Lightroom 6 as well as 4.4. Somewhat user error for sure on my part. But, all windows file handling was also directing to version 4.4. So if I clicked on a NEF file from windows it launched the wrong copy as well. I have since fixed this.
But, it is also interesting that this happens frequently. Does the installer have an option to remove the old version and import the data? Are people often wanting to keep the old outdated copy on their system?
But, that is precisely what the confusion was. Even worse, I did initially check for this, but still did not catch it due to different install folder name conventions that I or the defaults used.
Problem solved. Thank you! Minor thought about the installer maybe addressing this if it doesn't currently.