Redfish Virtual Media (vMedia) mounting fails consistently on Cisco B-Series (Blade) servers when provisioned via ACM / SiteConfig Operator, whereas Cisco C-Series (Rack) servers in the same environment work successfully.

When ACM attempts to map the discovery ISO, the CIMC reports a "Mount failed" error and the resource state moves to "UnavailableOffline." Logs indicate that the Redfish 'PATCH' method is not supported for this resource, and the subsequent 'InsertMedia' POST action returns a 400 Bad Request. Internal discussions suggest a potential communication gap between the CIMC and the ACM Hub through the Fabric Interconnects, but this has not been confirmed via packet capture. 

 ACM: 2.x (via SiteConfig Operator) OCP: 4.20 (Target) Hardware: Cisco B200 M5 Firmware: 4.3(6c) 

 100% reproducible on Cisco B-Series blades managed by Fabric Interconnects. 

Define a Bare Metal cluster using ACM SiteConfig/ClusterInstance.

Configure bmcAddress using 'redfish-virtualmedia+https' pointing to a Cisco B-Series CIMC.

Apply the configuration and monitor the node provisioning process.

Check Virtual Media status via Redfish API: /redfish/v1/Managers/CIMC/VirtualMedia/3 

 CIMC reports a critical health status with "Mount failed." Sushy/Ironic logs show: "HTTP PATCH... status code: 400, error: Base.1.13.0.GeneralError: The action 'Patch Update For Resource' is not supported by the resource." followed by a failed POST to InsertMedia. 

 The discovery ISO should mount successfully as a Virtual DVD, allowing the blade to boot into the discovery environment. 

Manual mounting via the CIMC Web UI works, indicating the ISO itself is valid.

The environment uses Fabric Interconnects for B-Series management traffic, which is the primary architectural difference from the successful C-Series deployments.