- connected UNO to WIN XP -> everything was fine
- connected UNO to WIN 10 -> everything was fine
- connected UNO to WIN XP back again -> couldn't establish a connection, the port field was grey without any possibility to modify it.
In my opinon there is no difference between the COM Attributes in WIN XP and WIN 10.
Did you upload a sketch that may have taken over the USB port or over write the bootloader while on win 10 ?
Did you connect anything to the board wrongly at all while on win 10 ?
You are mostly correct. To the end user there is little difference between the comm attributes under XP or 10 but in reality there is. (unless you just mean the attribute settings)
Maybe read THIS POST (https://forum.arduino.cc/index.php?topic=148850.0) first then add any further information.