To block external USB Storage devices using the command prompt type in the following:

reg add "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\USBSTOR" /t Reg_dword /v Start /f /d 4

Alternatively you could add it manually to the registry.

Please note: This is a “QUICK SUPPORT” article. The information contained herein is provided as is. As a result of the speed in making it available, the materials may include typographical errors and may be revised at any time without notice.  More than likely specific steps may be missing and it could very well be assuming you have an advanced understanding on how to complete certain steps.