Hi,
Not-monitored generally shows when some rules/monitors are not loaded in the agent using relevant MP's. Please go to Monitoring pane>windows computer>open health explorer for the agent & see what all rules/monitors are loaded or not in the agent.
Also, it's not clear if you have only installed the logical disk on all windows computer MP.
System Center Management Pack for Windows Server Operating System: https://systemcenter.wiki/?GetCategory=Windows+Server+Operating+System
Looking at the operations manager event logs will also give you an idea of what's wrong with the agent.
Try clearing the agent cache on the agent as well as the Management servers that the agents are reporting to.
Regards,
Amit