OK, first thing to consider. At the moment all you have to look at is your local network (anything to/from the internet uses a different set of functions and is mostly controlled by the router).
Writing/reading from your computer to the NAS requires that you have permission at the NAS and this is controlled by the CIFS/SAMBA settings and users/groups.
For the NAS to do the backup it requires permission to be able to read the hard disk of your computer. What can and can not do that is controlled by how you have set up the access control list and the windows built in firewall. A block by either the ACL or firewall will give you the error message you mention.
Where you find the setting for the ACL and/or firewall on win8 I do not know (every time we see win8 on a clients computer it is to replace it with win7).
You really do not want to hear my French