Yes, it's offline files and I've turned it off. Still, Cura beats on a network mapped drive I have every 5 seconds. Note in the image:
I did figure it out with help of some folks. It's the "Removable Drive Output Device Plugin". https://github.com/Ultimaker/Cura/blob/3.1/plugins/RemovableDriveOutputDevice/WindowsRemovableDrivePlugin.py#L50
The implementation has some issues:
It loops through all the drives. It should only be looping removable drives. There is a SO question about this.
And it shouldn't be looping at all. The app should register for notification of insertion/removal of USB devices to see if removable drives are present.
I did this several years ago to detect insertion/removal of USB headphones and the method was pretty reliable. I would contribute the code but it was in C#