You have the Symantec Endpoint Protection (SEP) client installed to a Windows device, and it is managed by the cloud portal (version 15). You added Device Control rules for USB mass storage external devices through a Device Control policy.
However, your USB mass storage devices don’t respond as expected. Existing rules pertaining to USB devices don’t seem to apply correctly.
A conflict exists between the following rules, which all apply to the Windows USB mass storage external device:
In such cases, those rules created under Windows > External Devices take precedence over those created under Windows > USB mass storage.
For USB mass storage external devices, create rules under the Windows > USB mass storage option.