Originally Posted by HarryT
With modern versions of Android that unfortunately doesn't work, because newer Android devices don't mount as a drive, but as a "portable device", which Calibre can't recognize. You have to use some other method to get your books to the Kindle app, such as copying the files to the "Kindle" folder using Windows Explorer, using the Calibre server, or using Amazon's "Personal Documents" facility.
Aha, I misread and thought it was a Kindle.
For these kind of problems I prefer to use DropBox.