This is more of a bug report but unfortunately I can't find a proper place to put it.
Steps to reproduce:
- Produce an invalid DNG (corrupt one)
- Introduce it into your catalogue (replace a valid one)
- Select the invalid image (or any group containing it) in Lightroom
- Select "Metadata -> Update DNG Previews & Metadata"
- Wait until the heat death of the universe
- At the universe's expiry, Lightroom has still failed to alert the user and acts as if the update is still in progress (progress bar is visible but empty)
- Lightroom shows the same error popup it shows for other processes like "Validate DNG Files", allowing the user to see which photos are problematic and fix them (by restoring from backup)
- Retina MacBook Pro 13", Late 2013
- Mac OS X 10.10.3
- Lightroom CC 2015.1.1