Missing/Corrupt EXIF Data

Question

I was looking at a folder of images today, and noticed that the camera data was not showing up and the images looked out of focus. Moments later the camera data showed up but the images still looked blurry (thumbs). Suddenly all the thumbs in the folder disappeared, being replaced by the .nef icon. I mean instantly, they all changed to the icon. I tried to open but got the message:
"Could not complete the request because it is not the right kind of document. "
So I tried to open it using DXO and it returned :
"This image cannot be processed since its EXIF data cannot be read or is missing".
Right click the image and the File Info is missing from the menu.
How does this happen? I have opened these images in the past. I can imagine the instant insanity should all my .nef's suddenly do this!
I am using Bridge running Photoshop CC on Win 7 64 bit. The .xmp data exists according to Win Explorer. There is no Bridge cache showing for the folder in Win explorer.
I am unsure as to whether this belongs here or in Bridge or???

Answer 1

I don’t think WFW would know about any outsider accessing the file on dropbox since it runs on your local computer.  If this is the first time you’ve used dropbox, it may have been asking for permission for dropbox to access your local system via the public network (there are local, domain and public tiers in the firewall rules) since dropbox synchronizes things between computers via the data stored in the dropbox cloud storage. 
In other words, I downloaded the file to my local computer using the public link you provided, and wasn’t logged in as you, so would have been accessing the dropbox cloud for retrieve the file but not be logged in as you, but I don’t think the WFW would know I was doing anything for that interaction because that is between me and the cloud, not something on your local computer.  Depending on how you answered the WFW question, you may have blocked dropbox from doing its normal things.  You can get into the WFW configuration in Admin Tools (or maybe Control Panel) and enable things for a particular program that you may have disabled by mistake.
Back to the corrupted file issue:  spot-check a few of the bad files on the backup, right-click Properties, and look at the Created Date and Modified Date, compared to the expected date the backup first would have copied them, to see if either indicates the date the zeros were written.  You might want to compare those dates on files that are ok, to see if there is some difference in pattern.  It is possible the backup software updates those dates to be what the source files dates are which might mask the backup-software's last-copy date, which is what you want.
If you can determine the date for the zeros and it is fairly recent, then the system event log might list something as having occurred. 
If there haven't been any viruses, then I'd say it was a transient condition, either in the circuit-boards or cabling or the hard-drive media, itself. 
Has the computer ever locked up so you had to turn the power off to get control back of the mouse? 
You can test the hard-drive media for errors by chkdsk d: /r or chkdsk d: /b (or the gui-scandisk equivalents) to scan for data errors--iffy media problems.  Move any virtual-memory swapfile off that drive if you want to test it while the computer is running.  Maybe do a plain chkdsk, first, one where you don't fix problems and don't test the media, which would find problems indicated with zeros in the directory-structure, first.  And if you find any of those, disconnect your backup drive in case the "fixes" corrupt even more things and that gets replicated.
A chkdsk that fixes media problems will likely take several/many hours.