Lose Music Signal

[simoncn]

The number of files on the HDD should not affect this.

Are you using the Ethernet LAN port of the RPi? If so, this will be an issue because the RPi uses the USB bus for its LAN connection. This means you are reading data from the HDD across the USB bus at the same time as sending/receiving LAN data, also using the USB bus. This creates a bottleneck and is likely to cause dropouts, as you are seeing.

If you are not using the Ethernet LAN port of the RPi, it is likely that the issue is with the CPU or memory capacity of the RPi.