Fairphone comes with standard Android 6.0, adding hardly any “FP-flavour”.
That’s part of the fair deal; not forcing customers to some non-free software, instead letting them chose what they need and are willing to install.
So, you are not complaining about Fairphone at all, but about Android.
If you are in fact complaining about unusal SD-card behaviour:
What about the questions posted by @Stanzi?
Have you been asked how to handle the SD-card (internal storage/mobile card)?
What did you do?
And finally:
There are quite a lot of different SD-Cards out there; some of them do cause trouble with some devices.
Take a look at this thread: