Q. How can I enable my Microsoft Zune portable media player to be used as a USB external hard drive?
A. By default, the Zune can only read/write content via the Zune PC application. It is possible to configure the Zune device to display in Windows Explorer with the following procedure:
1. Ensure that the Zune isn't connected to the PC and that the Zune PC software isn't running.
2. Start the registry editor (Start, Run, regedit.exe).
3. Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\USB
4. From the Edit menu select Find. Enter a name of PortableDeviceNameSpace. The match should be under the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\USB\VID_045E&PID_0710\.......\Device Parameters registry subkey.
5. Change the following Settings:
Set EnableLegacySupport to 1.
Set PortableDeviceNameSpaceExcludeFromShell to 0.
ShowInShell to 1.
6. Close the registry editor.
7. Connect the Zune and start the Zune PC application.
If the Zune PC application isn't running you won't be able to write to the Zune device. The Zune will show in Explorer, and by default two Zune entries will be displayed. To remove one set UseWpdPrivateInterface to 0 under the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\USB\VID_045E&PID_0710\....\Device Parameters\WpdMtpDriver key.
— John Savill