There are a few things going on behind the scene while booting and probably the first reason why you don’t see your music.
There is a second reason, I’ll come to that afterwards.
After the kernel has been loaded, the Volumio Initrd script gets control.
Initrd mounts the volumio_currents.sqsh from the volumio partition and mounts it as a squashfs (unpacked, readonly) filesystem on a loop device, let’s call it /mnt/static.
It then mounts the volumio_data partition and mounts the dyn folder which is in it, let’s call it /mnt/dyn.
With those 2 mounts, Initrd combines the RO /mnt/static and the RW /mnt/dyn into a new “overlayed” filesystem.
Overlay means: you can access the filesystem anyway you want. If something is changed somewhere, it will be in the dyn part, also the changes you think you made to the static part.
This on-the-fly filesystem is then used as the rootfs, which you will see as “/” after boot is complete.
Then control is transferred to the real init script in that rootfs and Volumio starts booting.
So coming back to your volumio_data partition. You will find 2 folders there, dyn and union.
union is the mountpoint for the overlay filesystem, useless to write anything in there.
dyn holds all the dynamic data that was added at runtime (which is actually the difference between the static and the overlays filesystem).
So basically, anything you write to the volumio_partition outside dyn from another system is completely ignored, so you do not see your music anywhere at runtime.
The second reason why you will not find your music is because mpd needs to be told to start scanning a folder structure.
Volumio does not do that for the rootfs it is starting from by default.
Volumio tells mpd when you configure an external (NAS) library.
When you attach a removable usb device, the system mounts it automatically and in the process tells mpd to scan the newly mounted USB device.
Now this is the only thing you could ask for as an improvement, some standard folder in /dyn/data to be used as your private music lib, to be mounted when it is present and (re)scanned after mounting. I’m not sure yet, how you got volumio_data visible in MS Windows
Why not put this as an issue in github.com/volumio/Volumio2 and see what the developers think of it.