I had a scare recently. I was planning to continue work on a photo in Photo RAW and I got an "Error 1001" trying to send the photo into Develop or Effects. The thumbnail in the Browse module was also black.

Uh oh. After a moment of panic, I thought about what might be the problem. Photo RAW was certainly unhappy with the file. Was the RAW file corrupted at all? Turns out, no. And, it turns out I was able to quickly fix the problem. Here are the steps I followed.

1. Show the RAW photo in the Finder (Mac) or Explorer (Windows).

In the Finder, I see the RAW file and the ON1 sidecar file. What's the sidecar? It's a copy of the non-destructive changes I made to the RAW image. This is an option you can turn on in the Preferences window. I enable this because it's a backup of my changes. It makes it easy to have processing changes follow the original photos when moving images across systems, too.

I noticed the RAW thumbnail in the Finder looked fine. That's a bit of a relief. At least the original photo appears intact. What about the sidecar file? Maybe Photo RAW is having a problem reading it? Time to experiment.

2. Shutdown ON1 Photo RAW

I closed the ON1 application. I'm about to monkey with a sidecar file "underneath" the application. I don't want to confuse the application with changes made outside of it. (For anyone that's moved folders of images without telling Lightroom will appreciate that! :)

3. Rename the ON1 Sidecar File

Photo RAW matches sidecar files and original photos based on the file names. If I rename the sidecar file, ON1 won't associate it with my RAW file. And that's safe enough because I'm not deleting anything. I add a "-broken" suffix to the sidecar file name.

4. Start ON1 Photo RAW

I restart Photo RAW. Good news! The thumbnail is back. And even better news! All of my non-destructive edits are intact. That saved me a load of work. 

So wait... how did those edits come back? Remember I said the ON1 sidecar file is a backup of the non-destructive edits. Photo RAW keeps its own copy of the changes in its internal database. Apparently, Photo RAW reads the sidecar if its present. By renaming the sidecar, Photo RAW used its internal database to appy the changes instead. Voila! My edits are back!

5. Regenerate the ON1 Sidecar File

I still want my sidecar backup. The sidecar files are generated when a processing change is made. I quickly open the Develop module and make a token change. For example, raise the Exposure slider and then Undo the operation. That's enough to refresh the sidecar file. And yes, I did stop and restart Photo RAW after the sidecar file was regenerated. All is good!

I've already been in touch with ON1 about this phenomenon. And we're running experiments to see if the problem is reproducible. It has happened to one and only one of my photos among thousands and thousands in my library. I believe this to be a case of the sidecar file being corrupted in some way. And not necessarily by ON1 Photo. It could be a bad block on a disk, cosmic rays... pick your poison.

I'm just glad the two-tiered method for saving non-destructive edits exists! It saved me from having to dig into my backups to restore files.