I had the same problem ... force the unit using the USB driver and you will solve your problem. To do so, Connect your unit and validate the drive letter with MS explorer. If you want to change the name, you can do this within Explorer.
Open MM. In menu, click on "tools", "options". In the new window, at the bottom of the left items, click on "portable/audio devices", on the right, click on the d_USBMass1.dll then click on the "configure" button (upper right of this window). A new window will open, click on the "Options" tab. On the left click on "device configuration" (the last item). Enter the name of the unit (Walkman or the new name you gave), select the appropriate drive letter from the drop down menu, enter a drive name if you want (not required). I then clicked on find device (it may not be required but for me it was). I was given 2 usb ids ... I tried the first one and it didn't work in MM. With the second one, my Z unit appeared and I could access it from the MM.
For me, this solved my synch problem (was taking eternity to synch 3 songs .... )