The Storage counter in Server health view indicates %Disk Time Counter in PhysicalDisk or LogicalDisk and is not measured by free/total space directly.It is a value derived by the diskperf filter driver that provides disk performance statistics. diskperf is a layer of software sitting in the disk driver stack. As I/O Request packets (IRPs) pass through this layer, diskperf keeps track of the time I/Os start and the time they finish. On the way to the device, diskperf records a timestamp for the IRP. On the way back from the device, the completion time is recorded.