system
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One of the things Avast does is scan files that are acccesed. When you do a defrag, Avast is “chasing” the defrag operation.
Why not just shut it down until you finish the defrag?
and besides, the defrag operation will run a little bit faster!
You can’t always get every program to work with eveything else.
I have had similar trouble running Disc scan with certain programs running. No biggie. I shut them down, do what needs to be done and reboot.
Good luck 