The root of the problem is that Google removed MSC support from versions of the Android operating system beginning with 4.0 (Ice Cream Sandwich). MSC - Mass Storage Media class mode causes the Android device to appear as a harddrive/thumbdrive. The Kindle Fire was based on Android 2.3 and so included MSC support. But the KFHD is based on ICS and doesn't.
If you are running Calibre on Windows, there is test mode (beta) for MTP support (which the KFHD supports). I don't remember offhand where in Calibre preferences the setting is stored, but I'm sure someone will post details.
Running OSX on my Macbook Pro, I'm dead in the water.... there's no MTP support for OSX.
Last edited by WillysJeepMan; 09-20-2012 at 07:27 PM.