Permissions for dialog service account

Hi All

I have a workflow that calls a file from a UNC path on the network. when I run this workflow as a background process it can access the path fine. but when I run the exact same process as a dialog it cannot access the path. I assume a permissions issue but cannot find any documentation on the service account that is used to run dialogs and how it would be any different to the async service we setup. I attempted to give permissions to the IIS user but still not fixed.

Any help would be appreciated.

Nick