USB Port busy, or not?
How to detect USB Port status connected with a USB Flash drive? (working or idle).
It seems no APIs or interfaces to get the status of a USB Port (working or idle).
As workarounds, it may work to force reflush cache by un-mounting and re-mounting volume or remove and rescan usb key device.
1. Flush disk cache by un-mounting and re-mounting volume
2. Devcon remove "USB\VID_XXXX&PID_XXXX" and then "devcon rescan"