The windows explorer bug was resolved by the recent flash update; it now mounts as a single object in explorer then shows both internal and external folders. The music is now displayed in internal as expected.
I’ll test the sync issue when I get home.
This is because a firmware update reverts the system back to it’s default USB setting, MTP (or maybe Auto Defect, which is the same thing). In MSC mode, the player and card slot show in Explorer under Drives With Removable Storage as 2 separate drives with their own letter designation.
Files loaded in one USB mode are not viewable or accessible while the player is connected in the other mode. You must’ve transferred the songs in MTP, and then somehow it got switched to MSC mode, so you couldn’t see them. Sometimes this is done manually by accident, or by someone unfamiliar with the settings, but this can also happen without your knowledge by leaving the player set to Auto Defect mode. The reason I have my own little pet name for it is obvious now, isn’t it?.