Can't access DLINK DNS320 from Windows 8.1 Pro notebook

I have a home network comprising a desktop pc running Windows 7 Pro which is connected via Ethernet to a technicolor TG587 adsl modem/router. A DLINK DNS 320 NAS is also connected to the modem/router via Ethernet. I aslo have a Gigabyte notebook running Windows 7 Starter and a Toshiba Satellite Notebook running Windows 8.1 Pro both of which are connected to the modem/router via wireless. The desktop pc and Gigabyte Notebook can both see and access the DNS 320, each other and the 8.1 notebook however the 8.1 notebook can not see the DNS 320, it can see the desktop pc but when I click on the desktop pc in windows explorer it asks me to "Enter network credentials". I have updated the firmware in the modem/router which initially enabled the 8.1 Notebook to see the DNS 320 but when I tried to map it as a network drive I was asked to "Enter network credentials". At this point in time I can no longer see the DNS 320 from the 8.1 notebook. I have "set the Minimum seesion security for NTLM SSP and disabled 128-bit encryption on the 8.1 Notebook. Where do I go from here? Can anybody help me with this?