I'm afraid no such issue is known/reported, so we don't have any information on that.
I'd say try to update to the latest internal version, as announced
here. There were quite a few problems fixed there (though as I said, not this one specifically - but some of the other fixes may solve this issue, too, don't know).
If the problem is present:
What happens when you mount the partition - do you get any error message? Any other symptoms?
Does it help if you stop avast! resident protection?
Does it help if you stop avast! self-defense? (Troubleshooting page of avast! settings)