>When you detach the board does the port number disappear in Device Manager?
Yes, Device Manager refreshes on detach and the port # (and the device) disappear from the Ports (COM & LPT) category.
>Are you running any other software that might be using the port?
The only other software is the laptop's Bluetooth system that shows up as "Standard Serial over Bluetooth Link (COM4)" and "Standard Serial over Bluetooth Link (COM5)". However COM ports 3, 6 to 13 and 15 to 19 also show up as "in use" when I try to do port assignment through Device Manager.
Is there any utility that can look deep into the system to show why ports are in use, and what uses those ports? Portmon is only somewhat helpful, it shows those ports that it thinks are in use.
I understand that the "USBSERxxx" IDs are the virtual serial ports, but is there rhyme/reason to how they are generated and assigned to Windows COM ports?