The D3 has several modes of connectivity when connected via USB.
1. PC Mode - In this mode the phone will create a single CD emulated drive for the purpose of installing the Motorola drivers. As part of the install you will see a browser window open directing you to the V-Cast site to download the latest version of the software. You don't have to install V-Cast. Once the drivers are install it will take a couple moments but then you'll see the D3 listed in the portable devices section of Windows 7. It will give you specifics on the number of voice mail messages, missed calls, text messages, the number of pictures on the phone as well as access to the SD Card storage. You can access the SC Card from both the phone and PC in this mode. You do not have access to the internal sorage in this mode. It will automatically set up a partnership in Windows Media player for syncing music to the device as well as allow you to sync photos to your PC.
2. Windows Media Sync - behaves pretty much the same as PC mode under Windows 7. It may behave differently under other OS's. I'm not sure why they included this as part of it.
3. USB Mass Storage - In this mode their will be no device shown in portable devices but you will have access to both the internal 16 gigs of storage as well as to the SD Card storage. You may have a drive letter conflict as I did that kept the internal storage from showing up. It attempted to grab the H drive letter onmy machine that was already mapped to a network share. If you only see one removable storage device after plugging in the phone check the drive letter mappings under disk management.
4. Charge Only - Just as it sounds. No removable storage is mounted in this mode.
Obvioulsy if you have the development setting for USB debugging turned on you will get some additional drivers installed related to those functions.
Oddly there is no longer a selection for unmounting the storage via the USB connections panel.