The new node "Music" of a portable USB device is not rescanned until a track is played there.
Even after a restart of MM it still shows the old properties. So if you e.g. change a rating on the device outside of MM, in Music node still the old one is shown, even after a restart of MM! Only if you play a track MM slowly updates the list.
Tested with d_USBMass1.dll and a USB stick.
In contrast, the Folders node is up to date when navigating to it.