An educated guess, as I never used a rPi as nas.
If you connect a USB disk to a volumio hosted pi, you bypass SMB, as you simply connect to a mount /mnt/usb/…
When you use a 2nd rPi, you suddenly add multiple components to the chain:
- Volumio SMB
- NAS rPi SMB
- NAS rPi USB
- Network
Hence, why I mention to get a reasonable priced NAS like the TR-002, in your previous topic.