Hi @fdavidg, I am so sorry for the trouble.
I don’t have a Windows instance up and running right now and this seems odd to me indeed. Are you by any chance running the npm process as the Administrator?
In general, Windows paths are weird in n8n and I’d suggest you avoid running n8n via npm. Instead, Docker is a much safer choice, allowing you to mount directories as needed and use standard Unix-like paths. For example in your earlier deployment using this command you’d use the -v C:\Users\fdavidg\.n8n:/home/node/.n8n option to make the C:\Users\fdavidg\.n8n directory to the n8n container available under /home/node/.n8n.
You can do this for multiple other directories as well, for example by adding another option -v C:\Users\fdavidg\Desktop:/desktop. In n8n, you could then simply write to /Desktop/myFile.xlsx.
If you do want to continue with npm, can you try removing your quotes around the file name and replace all backslashes \ except for the last one with forward slashes /? This has solved problems writing files on Windows in the past.
Please do let me know if this still fails, so I can try setting up a new Windows test environment and give this a go first-hand. This might take me a couple of days though.