For some reason I couldn't find the \system folder on the paperwhite, even though show hidden files/folders was checked. There was a folder amazon-cover-bug and I found the thumbnail in there and copied it to the thumbnail folder on my Scribe (which did show the system folder) and it seems to be working. I noticed the Scribe also has a amazon-cover-bug folder too, but I put the cover file into the thumbnail folder instead. I hope that was correct?
When I went back to the paperwhite, I noticed it was being identified as E:\Kindle, so I changed the nav bar to E:\system and it let me in and displayed all the subfolders. I found the thumbnail folder and noticed it also contained the missing cover, but it had the same name so I didn't bother copying that one over. Do I need the amazon-cover-bug folders? They both seem to be getting populated when I move books via calibre (which is always), so I'm thinking to just leave them alone. But I'm not sure which folder to copy my missing covers from/to until calibre gets fixed for the Scribe....do you think it eventually will be?
Thanks again for everyone's help!