There are multiple ways to transfer ebooks from calibre's library to a Kindle device:
- Send to Device: This toolbar button, and its releated drop down, appear only when a device is successfully connected and are used to copy the book to the Kindle's \document folder.
- Save to Disk: This will send a copy of the book to a designated folder on your computer. You can then copy the file to the Kindle's \document folder manually when the Kindle is connected to the computer via USB.
- Email: The Connect and Share toolbar button will give the option to email the file to your Kindle's account. The ebook will download onto your Kindle the next time you "Sync and check...". This method requires that you configure calibre (in preferences) properly so that it sends the correct format(s) to the email address assigned to your Kindle. You also need to access the Mangage Your Kindle page at Amazon and add the sending email address you use in calibre to the list of accepted addresses.
- Web Server: calibre has a built in webserver that will expose its library as a website. If you configure this service in preferences, and have a WiFi network at home, you can access it from the Kindle's web browser. The Kindle's browser will automatically place the downloaded book in the proper \documents folder.
Using the Send to Device function requires that the Kindle be attached via USB and that your computer recognizes it and mounts it as a Mass Storage device making it look like another hard drive. It also requires that the calibre version be new enough (there's a new one almost every week) to recognize the new Kindle. What version are you running?
Personally, I find myself using both Save to Disk and email interchangeably and never using Send to Device. This is primarily because my Kindle is rarely connected, or even in the same room as my computer, when I decide to send a book to my device. I'll move it to the device later, either when I sit down to read if sent by email, or the next time I connect the Kindle for charging.