problem with setting to monitor disk Usage

How can configure correctly disk usage. To analyze my hdd network? I tried entering parameters but not analyzes the chart:

if(hdd_usage()>0, 1, -1)
if(hdd_usage()<90, “”, “Disk usage > 89%”)
hdd_usage()
%

help me please
I’m italian

Hi
I have the same problem.
My probe is always at 0…
Looks like some error on the parameters of the probe…

Any help?? Anyone that can double-check, please?