what exactly are you trying to achieve?
You have an external HDD connected to your Pi and you use this as your media source for your Squeezebox Server. If you connect a HDD and it is not shown in „External Storage“ on „Filesystem Mount“ in Max2Play Webinterface, it might be that the Format is wrong or maybe the power supply is not strong enough to power the PI and the USB system with the HDD connected.
But then you want to connect to a „Filesystem network path“ which means that the source is on another computer/NAS.
We recorded some videos on how to connect the HDD and also how to mount network storage to use it as media source.