The partitioned drive is used to store updates for your pic/laptop , In shirt you pic / laptop will download updates but until you update it needs to store them somewhere . When you have updated the partitioned drive will delete .
Local Disk (Z) on my laptop is 256 MB with 169 MB free. When I double click it, it tells me I don't have permission etc. When I right click and select "Properties" it opens the System Information Page. This may be the case for you also. I did say may. Of course when you right click (C) Drive and select properties it also takes you to the System Information Page.