If NodeRed is running on Ubuntu it is running on Linux - and you cannot access files out of your virtual machine. I have no experience - but you can also use the exex node - and you may have the same problems. I dont think that it is possible to access outside of the portainer.
Either you store your csv in the local context of the portainer or yes you share a folder on the windows machine and mount it to the Ubuntu system. I guess that you have a general problem to access files outside of your vm.
