I'm an SQL newbie but very familiar w/ os level scripting, WMI can be costly be careful where and when you use it! . Your script is great btw, i use a similar script locally on some servers to monitor and kick out to an opensource system monitor however i've found that certain WMI queries can be taxing on systems.

Using a native to the db call on the system the db is running on in theory should be less costly cpu/resource wise than using wmi. Then there is wmi security, network transport security etc, all of which you avoid by using the db system service to run the queries.

On a semi-related note, did you use wbemtest to find the properties of the disk device? I've been having some trouble locating particular wmi attribute detail. Do you know if sql publishes info via wmi?