Disk_Drive is the DCE device that handles all access to the optical drives attached to a media director.
It exposes a series of commands which read from the drive, facilitate ripping, and expose something to the media database (presumably for viewing) ... right now, we detect for DVDs and Audio CDs, then provide media identification services for it to get cover art. Once the media is identified, it is presented to the system so that it can be viewed from the file list.
You can do a check-out of the code, here, via SVN:http://svn.linuxmce.org/pluto/branches/charon-merge/
the source for the Disk Drive device, is in src/Disk_Drive/
Disk_Drive also uses a few other bits inside src/ to do its work, but this should become evident upon reading the code.
LinuxMCE is a very intricately designed system, and it will take some time of reading the source code to understand the system. Read the code, look at the DCERouter logs, and look at the corresponding logs for devices in /var/log/pluto. Watch how the system works...