In the current version of avast!, there is a small bug regarding the error message. However, in the upcoming update the problem will be fixed - so it will be possible to avoid the error message by putting the whole partition into avast’s list of exclutions (as E:). I know you probably don’t want to have this partition at all, but there are some people experiencing the same problem e.g. with unformatted partitions they want to keep for some reason.