That's a corrupted image. When you export the DNG to a JPEG on your hard drive, is the resulting file also containing that blemish? If so, then the DNG is corrupt. If not, it's your preview that's corrupt.

Did you convert the files to DNG on import, or did you do it afterwards?

Hal

Probably not. It's the same process.

Late-model DNGs contain a checksum that alerts Adobe programs if they go corrupt after they were created, and your files don't do that. If they did, you'd get an error message when you tried to us the DNG. That says to me that either the original raw file was corrupt or that the corruption happened during the creation process. Do you still have the original?

Hal

So try importing the original file (without converting to DNG). Does it have the same blemish when viewed in Lightroom?

Hal

Good. I'm curious to know if using Lightroom to convert that NEF to DNG causes the problem to happen again.

Hal

I'd say so. It's good to keep backups of all your images.

We're left with the question of how it happened, but I'm fresh out of ideas. Does your computer have a memory test application that you could run? If so, you might want to run it. Intermittent problems can be caused by hardware glitches.

Hal

I have seen this photo corruption issue cropping up regularly on forums (it also happened to me years ago). The worst thing is that it is a silent process: if one doesn't happen to look at a particular photo one may never know that it is corrupted.

I put a feature request forward to implement an automatic photo integrity check into Lightroom:

http://feedback.photoshop.com/photoshop_family/topics/photo_integrity_ check

Please feel free to add your vote.

In the situation a year ago, was it media failure or something else that caused the “corruption”?

Hal, my understanding is that this check is done only when working on an image (DNG). So if the image is left alone - say for 6 months - Lightroom won't provide any warning, is that correct?

1- So it looks like a very quick implementation of the data integrity check would be to do the check on demand for every DNG in the catalogue.

2- At a second stage, this function could be extended by creating checksum for all types of raw files and also jpg.

Thanks for the link, this is a cumbersome workaround, but better than nothing.

Maybe a plugin could be written that implements the SDK function to validate all the DNGs in a catalogue. But that still leave the issue for other raw files and jpg.