I had exactly the same problem, and fixed it by reflashing with the eMMC rom, exactly as explained in this thread. After that I could browse the device with the file manager, and it said 30 GB free or something. But then I found out I also had another problem, because my microphone seemed broken. And I'm not the only one as you can see in this thread. I returned my phone to Nokia today, hoping to get a new one next week.