Does the system process slow down my Database access?
One of the common disk usage process on a windows 7 machine under the System process is Superfetch.
You might want to look into disabling it disabling superfetch.

The reason why it's likely Superfetch is because you're seeing 


Only reads
It's not your OS disk
You're running windows 7, not a server version
more slowdown as the amount of files increases


If that doesn't help Sysintenals' Process Explorer might help you get a better idea of what's going on. sysinternals