I think what you misunderstand is what I was solving for - to make sure LR/ACR will not change the file, which is what at least some people want from a DAM system. I wasn't getting into the issue of under what circumstances LR should or should not write a sidecar file, or trying to write a sidecar file. Personally, I'd say that refusing to update the DNG and giving an error message is reasonable behavior, but that's neither here nor there. The point is, (a) the file wasn't modified, which is the behavior I was solving for, (b) LR is still doing its thing, (c) and there was a clear message telling you that what you tried to do couldn't be done. If you think LR should write an sidecar file under those circumstances, that's fine - personally I'd be quite happy with that behavior as well, suggest you file a bug/feature request with Adobe.I'm certain I didn't misunderstand as it wasn't what you said:
Your statement is that setting the files to locked will cause Lightroom to use .XMP sidecar files. This is not what happens. XMP sidecar files are not generated for it to use, the write does not succeed. The original files remain unaltered, but that would be the case anyway if you didn't tell Lightroom to write the metadata to them.
I did the example on my test system for which the ONLY account is the administrator account, that answers the question as to whether administrator privileges has any bearing on the locked-file behavior: it doesn't.
You seem to be mistaking this behavior with Camera Raw's behavior, or perhaps Eric Chan is. When a DNG file is locked in the file system and you open it with Camera Raw, apply changes, and click Done or open in Photoshop, it writes .XMP metadata files to save the changes, presuming the Camera Raw preferences are to save changes in .XMP sidecar files rather than its centralized database.
Lightroom, although it shares a good bit of the Camera Raw code for the image processing, does not work this way.
And yes, if you read my original post, you will see that I started it with "LR/ACR". Apologies if I wasn't consistent in saying that I was talking about both all the way though.
Sandy