An app can only upload to an external wi-fi device if the developer built that feature into the app. So, in most cases your notes and other text document will not upload directly to the wi-fi device. However, a 'good' wi-fi device will come with a good support app. This app should be capable of recognizing most document types and show up as an Open In option for apps with those types. What that means is that you can 'probably' copy the document to the support app using Open In, and then save that to the Wi-Fi device. A process that will involve going back and deleting the document afterwards if you don't want two copies. So, when researching your wi-fi device also research the support app that comes with it, and make sure it supports all the document types you want. Apps that do not support Open In, like the Notes app, will require more indirect methods if you want to save their content on your external storage device. All this is from the perspective of someone who has not actually purchased or used one of these devices. I've read about them, and I now how apps can and do transfer documents for other similar (mostly cloud storage) solutions. None of this guaranteed to be true.