VirtualBOX 不能mount优盘，移动硬盘解决方案

The Solution (basically nayasis' solution with the second driver installation added):

1. Safely unplug your USB devices from the host if any are plugged in.
2. Open VBox then close it again to release any USB devices that may still be captured in the VM.
3. Delete the UpperFilters entry, if it exists, in the host registry -
   • Open the Windows registry: Start (right-click in Win 10) > Run and type regedit.
   • Navigate to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{36FC9E60-C465-11CF-8056-444553540000}.
   • Delete the UpperFilters entry if it exists.
   • Note: This entry did not exist on my system.
4. Manually re-install VBox USB drivers on the Windows host -
   • Open folder C:\Program Files\Oracle\VirtualBox\drivers\USB.
   • Open subfolder device, right-click driver file VBoxUSB.inf, and click Install.
   • Open subfolder filter, right-click driver file VBoxUSBMon.inf, and click Install.
   • Reboot the Windows host system just to make sure the drivers are loaded.
5. Plug in your USB devices.
6. Open VBox and run the VM.
7. Click any devices in the VM's Devices > USB menu list, and they should mount OK. (Hovering the mouse over the USB icon on the VM's status bar in the lower right corner of the VM will show the USB devices attached.)

The Probable Cause

Win 10 had crashed and recovered itself during the original VBox install, and the installation was not completed. I had not disabled Norton Security and suspected the antivirus had caused the problem. After rebooting, I closed the wifi, temporarily disabled the antivirus component of NS, and successfully re-ran the installer to completion. VBox seemed to run OK so I didn't give this a second thought. But apparently the USB drivers had not installed properly.

Hope this helps someone. And special thanks to nayasis for his post