And how did you solve that ESPEasy returns an answer statement? I have the same error, but the functionality is working. I want to avoid unneccessary error messages.

I think the RPi Zero W is still a little bit overkill. Better is an ESP8266, something like a Wemos D1 mini Pro. See http://www.esp8266thingies.nl/wp/

Can you tell me how you got the decimal part correct? My Nas has 2,6 TB free space but domoticz sees it as 3 TB

[Edit]
Found it, need to use a "Custom sensor"