Update.
Idly googling this issue I find a 15 page thread on problems getting windows and kobo to play nice. All sorts of fixes suggested.e.g install kobo desktop again, go via a usb hub.... I wonder if the issue is not with my device port after all but may have been triggered by installing windows fall creators update ?
https://answers.microsoft.com/en-us/...-kobo-ereader/
I don't have kobo desktop installed, I could go get latest version, from kobo I assume.. Theory is that it may install newer drivers ?