The _introspection index is only viewable by admins. It is available for any Splunk instance, regardless of OS.
If you don't see anything from your query, try a broader search like "index=_introspection" and check to see what hosts appear in the results. Perhaps your host name is wrong.
Many of the reports in the Monitoring Console (formerly the DMC) are based on the introspection data. Hopefully, you have set up the MC for your environment. You can see a lot of the disk usage information there as well.
Appreciate your help on responding to my question. But as I am pretty new to this, I am not clear on what does it mean by MC. I am not sure if that was done by our Admins. I am not sure if I am an admin. Is there any other way to find out the disk usage other than the introspection?