Well, but Avast may or may not be the "problem". The OS itself may be "scanning" (not in the sense of the antivirus) the disk, for several reasons. Just to give you 1 of many examples, it may be some index for Windows search. There are many others, depending on the specific OS and other tools and features installed.
The question would be if this happens only when you "connect" the disk for the "first time after the last boot", or it keeps happening also after, say, 10 minutes, or if you disconnect it and re-connect it again after an hour without rebooting in between.