Hi, HolyRoyal,
The IDM policy cannot be implement to DLP Agent to trigger block action.
The document content of the file that you copy to USB will be send to the Endpoint Server for IDM policy detection. That's mean, is you copy a file inside your IDM sources to USB, the incident will be generated in Endpoint Server and be written to DLP enforce, but, the block action of your response rule will not work.