I am a bit confused as the diagnostics do not show disk1 as being anywhere near full.
the shares are set to use highwater allocation method so with 1x16TB and 2x8TB drives I would not expect disk 2 to start getting used until disk1 is down to 25% free., which is what it looks like what is happening.
the syslog in the diagnostics contains continual USB Disconnect messages which makes it hard to look for other errors. Any idea what USB device that is, and can it be removed?