I have a couple of dng files which Lightroom (3.4.1) is showing as unsupported or corrupt. They were converted by the dng converter and I have, alas, thrown the original CR2s away (to save space - dng files are considerably smaller - and dng conversion usually goes without a hitch). There is not a blemish on the previews and I can enlarge them and manipulate them all I want but I can't send the files to Photoshop. Is there any way I can repair these files? (Win 7 x64)
Sorry, they're dead.
There's a plugin that will extract your Lightroom preview as a jpg. It won't get your raw files back, but the jpegs ought to be good enough to print.
Best practice says that you need to keep at least two copies of your originals. On separate physical devices.
p.s. That website I referenced also points to a more full featured program that does the same thing.
If these DNGs are a bit smaller than the rest of the DNGs they are likely corrupt and extracting the JPG preview, somehow, seems to be the most likely workaround.
Is your ACR updated to version 6.4.1 or something less?
What do you mean about being able to manipulate the previews, do you mean in Windows Photo Viewer or in LR?