The other topic we think there is some confusion to say the least in what is going on (is also avast 4.
so I believe unrelated to this question.
We will need a lot more info, OS, CPU, RAM, Sensitivity, if archives were selected (in fact and change to default settings for the Full scan), the amount of data on the two HDDs. So as you might see we can't answer your question without more info.
The first few times you run avast scans it is also building up its Persistent cache database, this is used to speed future scans for files either in a white list, etc.