There have been some developments in the 'Sony thumbnail saga' today. The next release of Calibre should bring big improvements.
The intent is, and always has been, that:
- If you check both Upload separate cover... AND Refresh separate covers... Calibre will resend cover thumbnails EVERY single time you connect the reader to Calibre, i.e. your Sony thumbnails should always be in line with calibre, even if you change a Calibre cover after the initial Send-to-Device. No need to resend the book to the Sony. I believe you also need to have Metadata management set to Automatic in Preferences - Send to Device for this to work.
- If you only check Upload separate cover... Calibre will send cover thumbnails only when you Send-to-device. So if you change a cover in Calibre after the initial send to Sony, you would need to Send-to-device again for the Sony thumbnail to be updated.
- If you check neither Upload nor Refresh then Calibre will not send any thumbnails to the Sony. The Sony will extract its thumbnail from what it finds actually inside the book.
The intent has not changed.
However, last night I found a few problems with thumbnails when sending to SD card rather than internal memory. Developer chaley has fixed these problems today. We also may finally have the solution to the "non-sticking-thumbnails-after-first-send" mystery. All of this, hopefully in time for the next Calibre release.
My advice would be to not spend much time trying to sort out your Sony thumbnails until we have the next release. Life should become much simpler and more predictable.