By default you will be alerted if any filesystem runs out of space, regardless the mount point or the driver letter. But maybe you don't care about drive
C: and you want to be alerted about drive
D: getting full.
Disable the default rule
Agent - File system free percent (any disk) .
Go to the host list and select any host that has a drive
D: attached. Open the metrics section of that host, navigate to the fill-level of drive
D: and click on the wand icon.
Now you can create a rule only for a specific disk identified by a drive letter or a mount point. You can choose wether you create that rule for all hosts or just for the one you used a "starting point" for the rule creation.