Hi Carl, I'll do my best with the questions:

Has the MD600 been used as Sooloos Core before in your system?

No. I recently purchased this from another Hiker who had it set up as a core on his system with no probs. I simply ran the restore through Control:PC to upload my backed up music from my dead C15.

Is the MD600 on the same subnet as your other equipment?

All equipment starts with 192.168.1.xxx Not sure if this indicates they are on the same subnet but happy to do some digging if you can guide me

Is it using DHCP assigned or fixed IP address?

DHCP

Have you checked for IP address conflicts?

No IP addresses on my system are duplicated if that answers the question - all are set as DHCP.

In the Sooloos Config program the MD600 does have 3 separate entries of 192.168.1.74 but only the entry stating "4TB media" can be accessed to enable core, storage and DHCP - may not be related to my issues.

What happens if you point a web browser at the MD600?

Pointing a web browser at the MD600 loads the webclient which works fine.

I tried turning off the firewall in windows defender and the router too.

Cheers,
Simon

Main: MS600, G61R, DSP3100Vx3, DSP33x2, SW1600, Samsung 65JS9500 TV, Sony X800, Wireworld Star*light video cabling, Isotek Vision, Isotek Elite/Prem mains cabling.
Office: Roon ROCK on NUC7i5, Allo USBridge + iFi PSU, Prime Amp/PSU, Fostex TH900.
On the Go: Surface Pro, Explorer2, Sennheiser Momentum M2
Smyth Realiser2 on pre-order :-)