There seems to be two ways to interpret this question, so I'm going to tackle both.
First: Can I monitor the LEM's disk space?
Yes! There's a couple ways to do this. If you SSH into the LEM, under APPLIANCE you can run a DISKUSAGE command to get an immediate idea of the LEM's current disk usage.
Better yet, there is a template rule that is shipped with the LEM called "SolarWinds Disk Warnings." This rule fires if any of the LEM's partitions gets over 90% full, and the default action is to send an e-mail. Paired with another template rule, "Appliance Database Down," you can get alerts about something going wrong in the LEM before it's a big issue, and a timely call to support can get you fixed before anything is really broken. Both these templates are tagged under "General Best Practice" in the Build --> Rules section of the GUI.
Second: Can I monitor disk space with the LEM?
It's important to understand that the LEM is event driven. If the system can generate an event that a disk is full, the LEM Agent can capture that event, and a rule can be constructed to take some action, like send an e-mail.
In Windows, there is such an event. It's ID 2013 in the System log, and it looks a lot like this in the LEM:
Important: There's no way for the LEM to set the threshold that fires this event on a disk. That's a setting in the Windows Registry, and by default it fires when the disk reaches 90% full (or less than 10% free, depending on your point of view). You can change this, but it'd be done in the registry on each system or through a GPO.
There's a template rule for this as well, "Windows Disk Nearly Full." It's under General Best Practices as well.
Thanks for the response. I was looking for option B but both are very helpful.