Type the server name in the top right search box to select the server you want to view.
Select the "Screens" link for the server you want to view.
Make sure "Disk Overview" is selected, and choose your time period.
Disk utilization is shown below.
An overview of disk space for a specific disk on a server, can be accessed via the graph function.
Monitoring - Graphs - Host Group - Host - Disk space on...