This siutation is encountered in Azure Function where we want to have telemetries for available space for temp as well as home. It is a general issue of getting available space for mounted folder.
See the result, notice the difference between using different ways to get free space for the same folder? Why?

Assuming temp folder location: c:\users\abc\temp, disk c has 1G free, and temp is mounted from another volume that has 5G.
This gets the free disk space for the driver, not for the mounted folder - 1G: