Hi Remy-
Thanks for the sample test.gh file.
I’ll just echo @nathanletwory 's comment here. In your test.gh, your paths are to folders, not files. The paths need to be files.
If that doesn’t work, then I suspect it has something to do with the creation of those folders using your C# script. As a test, if you create a folder using macOS’s Finder (not a C# script) does the File.Copy
operation succeed (using a fully-expanded path to a file)?