Does the router have NAS like capabilities for connected devices? In order to connect, Sonos uses SMB (v1 for S1, v2 or v3.x for S2), and not standard windows connection protocols. If the router doesn’t support NAS like connections, it likely won’t connect, because the ‘type’ is incorrect. You’re likely going to have to delve in to the router’s manual to see the type of sharing it does. It’s also possible, depending on which Sonos OS you’re using, you may need to update/change the settings. 
 

Frankly, it’s possible this will fail, if the router doesn’t include SMB. Some do, many don’t. Especially for USB connected drives. Sonos is expecting a complete NAS system.

Thanks Bruce. I see your point.

However, the router does support SMB2 and the USB drive attached to it, is accessible from other OS’s.

There is no mention in the router’s manual regarding any NAS characteristics of the USB port.

Looking at other posts on this subject  I can see others are having similar problems.

Elgreco

If the router supports SMB 2, then it’s running some version/portion of NAS software, to allow access. Windows uses a different access method, which is why it can ‘see’ the data.

Note that Sonos S1 can’t use SMB v2. You don’t specify in your post which OS the Sonos is running. I’d be following those other threads you’ve found regarding the issues, as well as calling Sonos Support to discuss it.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.