I just upgraded my projects to RH 2020 Update 6. Using RH 2019, I had set up my images to display as thumbnails in my Responsive HTML5 output (Indigo layout). For each image, RH generated a separate _th file that it displayed as the thumbnail.
With RH 2020, there are two thumbnail options in the preset: Create Thumbnails for Images and Style Existing Images as Thumbnails. I have generated the output both ways and I don't see any difference. I expected "Create Thumbnails for Images" to generate the duplicate _th files as in RH 2019, but I don't find any such files in output folders. I can't image what it means by using "style" as a verb here. And yet either way, it works.
What is the difference between these two options and why would I choose one over the other? I did look this up in the RoboHelp user guide, but it just explains them using the same words that are on the screen.
If your project was previously on a network drive and you were generating to a network drive, that could definitely lead to issues. The specific network setup and your connection to it can cause slowness and also micro-interruptions, which could mean files aren't generated to the right place (or aren't saved or only a partial file is saved). Additionally, network paths frequently can be extremely long, and long paths also prevent files saving - this is a Windows limitation, not an RH issue.
I personally always keep my project on my C drive and in a folder right off C, not inside the Documents folder. This prevents network issues and long path issues. If I need my output on a network location, I set up a publishing destination and use that to copy the output to the required place - there's much less chance of something going wrong when the only thing happening is copying files, and I believe the technology is designed to handle network dropouts better. (To back up my source files, I manually zip the entire project folder and copy the zip to a network location. )