Cannot store or delete any images on my Fairphone 2 anymore

If the filesystem is damaged the system also sets it to read-only.

But as the sd card is used as internal memory in one block it’s not so easy to separate it. I am not quite sure how to check the internal filesystem (without rooting the device) and how it can be performed to separate the added sd card from the system again to remove it to get it re-formated and checked in a usual computer, maybe even replaced if faulty.
Maybe someone else here knows how to get it done. A full clean reinstall may help, but this will not assure the sd card being undamaged.