Yes! .. wondering WTH!!(this is a family friendly website .. the actual reaction had an 'F' in there when I saw the space invaders screen!)
Anyhow, in the end it was easy to work out what the issue is, and why I saw it on some files(D5500, D300, D800E and D70s) .. but then not on others.
Originally I viewed/opened raw files I've already edited.
Like I said in my last post, the initial thumbnails are displayed as per the embedded jpg preview files in the raw.
When I edit with Nikon software, the edited raw file will get the embedded jpg preview files re rendered to reflect the edit made.
Unedited files just look like the original images straight from camera .. usually flat and colourless, or incorrect WB .. or whatever.
So the problem in DT turns out to be, that if the NEF file is edited by Nikon's (older) software, it has trouble reading the NEF file, as Nikon's older software does update the internal XML type data in the NEF file directly .. and update the embedded thumbnail images.
So I can see the thumbnails fine in DT, just when I load those edited images I see those weird white images.
The more accurate description of what DT is doing is not over exposure, it's just a convolution of the raw data. When exposure compensation is applied, or a massively under exposed image is loaded in DT, the image goes all funky looking.. weird purple/magenta squiggly lines and patterns and stuff like that .. and the massive white-space.
So I tried a test:
Selected a random image in ViewNX2, edited (just WB), saved and then reset the image back to it's original style again.
(the WB edit was a wild, totally goofy value .. used a WB setting of TN-A1 which has a Kelvin value of approx 2400K, then tweaked that value down to 2100K. moved the tint slider down to -12(Magenta), and it rendered the sunset image a deep purple!)
Opened the edited raw file and as expected, DT(via it's lighttable module) displayed the thumbnail with the same purple hue. Opened in DT and as expected the image had the weird rendering.
Closed DT, back in ViewNX2, reset the image back to original WB.
DT then took a bit of time to create a re rendering of that 'purple' version of the raw file, I guess a database syncing issue, and after a while displayed that raw file with the correct WB setting. Tried to open the rest raw file but DT rendered the same weirdy rendering with the white-space and purple crazing again.
Not only does it incorrectly display the raw file in terms of its raw data, but it also assumes that it's a landscape orientation, when it's in fact a portrait orientated image.
So once some of the raw file has been manipulated, it has trouble reading that data for some reason.
So far haven't seen any issue if that raw data manipulation is restricted to keywording/ITPC or ratings/labelling data.
*note that I prefer my raw data(original or edited) to be contained within the raw file .. ie. ala Nikon's (old) software style. Nikon's new software uses the more common method of reading external XML data files for each image, which I don't like.