[How to do instruction] Access to local files through Windows

[TamedShrew]

You could use the one pi with your USB drive attached and SMB File sharing turned on as a NAS.

Create a Music source on the other Pi's that references the NAS Pi.

After copying new music files to the NAS Pi
Menu, Update library
Then
Menu, Players, Action = Update library, Submit

All the Pi's will be up to date with respect to the Music collection

That's a great feature. I think I'll probably keep what I've got for now as they sort of act as a backup of each other and one of them is in a bit of wireless black spot in the house. Something to bear in mind for the future though.