WD MyCloudThe initial thought was “it’s another ‘SMB1 is disabled’ causing connectivity problems” problem.

Except it wasn’t.

The issue was that our customer reported that they could no longer connect to their NAS device.

With Windows 10 v1709, Microsoft disabled Guest Access.  In their words:

This change in default behavior is by design and is recommended by Microsoft for security.
A malicious computer that impersonates a legitimate file server could allow users to connect as guests without their knowledge. Microsoft recommends that you do not change this default setting. If a remote device is configured to use guest credentials, an administrator should disable guest access to that remote device and configure correct authentication and authorization.
Windows and Windows Server have not enabled guest access or allowed remote users to connect as guest or anonymous users since Windows 2000. Only third-party remote devices might require guest access by default. Microsoft-provided operating systems do not.

Guest access in SMB2 disabled by default in Windows 10 Fall Creators Update and Windows Server 2016 version 1709

For the small number of end users who will need to connect to a third-party NAS, we’ll probably manage it via exception.