Hi @Geoff_Bishop,
I’m terribly sorry you’ve had such a frustrating experience with this shared folder.
Just for due diligence, do you happen to have a Firewall enabled?
Several Windows users with Roon have encountered issues with network share and resolved the by creating a new local account on their PC, and using that to connect to the share. Make sure you have added the new account to the sharing list. Guest access is no longer available in Windows by default. Here is a useful thread describing their solution: Backup to network share
Do you also have permissions set to Read/Write in Network Access?