Standard .NET file access is restricted to prevent rogue access to your files, this means you can not use System.IO.File or variants directly.
Instead, s&box provides a BaseFileSystem for several virtual filesystems that can only access files within specific game directories.
FileSystem.Mounted is an aggregate filesystem of all mounted content from the core game, the current gamemode and its dependencies. It's a combination of the following directories:
Any time you try to read a file it will search each of these paths.
FileSystem.Data is a place to store user data for your game.
FileSystem.OrganizationData is a place to store user data across several games in your organization.