Ok so Content -> Media -> Media libraries folder is the default (i.e. not set). According to the tool tip that default is ~/{site name}/media.
We don't want to hardcode physical paths like C:\TestFolder.
We just want to create an IIS virtual directory for /media which points to a physical location outside the website location, and would expect Kentico to use Server.MapPath() to determine the physical location when it is writing files to that folder.
Regarding this scenario we refer back to the post made by ks-adaptdev in 2014 in the provided link:
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But, configure the path "/Libraries" in IIS to be a virtual directory, which maps to the physical path of c:\files.
Again, we'll configure Kentico to use "~/Libraries". Upload a file, and you'd expect it to end up physically saved to disk at c:\files with the url /Libraries/{file}. This would be standard .net behavior assuming the code written to save the file calls Server.MapPath("~/Libraries") to determine the physical location. But this isn't what is happening.
Kentico ends up ignoring the fact that /Libraries is virtual, and saves the files physically on disk at c:\wwwroot\site\Libraries. This is not what one would expect. This means your files aren't saved where you wanted them and the Urls to them 404 because IIS is looking for the physical file at c:\files{file}.