Port Multiplier (PM) has two different structures, one is called Command-based switching and the other is called FIS-based switching. Here is a link of a diagram:
To see multiple individual HDDs simultaneously, you need to look for "Port Multiplier with FIS-based switching". Command-based switching only allows you to toggle between HDDs.
As far I know Intel's ICH9R and ICH10R both don't support FIS-based switching PM.