There are multiple options to do this.
i. You can run Command Prompt from Blue Prism and then using the below command save the drive details to a text file and then read the text file. You can convert the figures to GB if you would need it that way.
wmic logicaldisk get size,freespace,caption >c:\DiskSpace.txt
ii. Write a vbscript file (and call from BP) to get the drive details and save it to a text or excel file. Lots of them are readily available over the web.
There could be many more such methods.