When a full scan run like the name implies it runs a scan against everything. For macOS mounted drives can be found in the /Volume directory where things such as your own SSD / HDD can be found but also USB drives and mounted drives. The other thing that can be found is your Time Capsule NAS and local snap shots.
As I too try to find some reasoning as to why my Full Scans seem to get stuck on arbitrary files I stumbled upon the realization that when I kick off a Full Scan my NAS drive IO spikes as Kaspersky tries to perform a full scan on all of my backups. I’ve unmounted my NAS drive and tried again but is this behavior intentional?
I have Kaspersky Internet Security but I figured the anti virus part of it makes more sense to be placed here.
My other searches are yielding no results so I’m reaching here. I have not contacted support.
//Mod Note: moved to the correct section.
Best answer by kelvixView original