Thank you Glenn.

What about point 1? Any suggestion or solution for that?
1. I have the script that I have specified per MB is "$MB = 1048576". See detail of my script at the bottom of this post.

Following are the results:
Drive C has 130GB of free space, it output as 130.159145355225
Drive D has 728GB of free space, it output as 728.70979309082
Drive E has 3.6TB of free space, it ouput as -1958.04564285278