How to enable usb file transfers?

[BertG]

Maybe you find the solution here (or rather in the links on that page):
https://support.fairphone.com/hc/en-us/articles/214620623-How-to-connect-your-Fairphone-2-to-a-computer-Windows-Mac-and-Linux-
As I don’t use Linux, I have no idea what to do on this behalf.
Also you might find the information in this post helpful on selecting between internal storage and portable storage for the SD-card:

Access to apps on sd card not possible with android 6

That is not what the original post is about. @piccolini refers to apps that were transferred to the micro-SD card. In your case, this is the prompt you get when Android 6 first sees your card. Selecting Portable storage will make it act as it was before, with Lollipop. That means that the card can be read by other devices if you take it out of the phone. Selecting that option will not format the card, the data will not be deleted. The new mode Internal storage will format and encrypt the ca…