@dave maden: If you have another computer on hand, try there (not necessarily within Calibre, just seeing if you actually can get it to pop up reliably in the Explorer is a start).
You might also try another USB cord, and after a full restart of your Kindle ([HOME] > [MENU] > Settings > [MENU] > Restart).
@books55: Nope, not relevant. Although ADE is for ePub files, so I really hope you're not just putting epub files on the Kindle, because, yeah, obviously, that won't work

.