After I rebooted my Xperia Z2 running Lollipop 5.0.2, the micro SD card is not indicated as being mounted in Settings → Storage. I see a button to mount it, and another one to erase it, but it is accessible from FX File Explorer and I can read from it, and write to it without a problem.
Here's a screenshot of the Storage screen showing the card unmounted : https://goo.gl/photos/VdtZNy8uN52q9N9e7
If I remove it and put it back in while the phone is still on, it shows up just fine in the Storage (I see a button to unmount it, and another one to erase it).
This makes me think that something is going wrong at start-up, that prevents it from being mounted properly.
I tried several new micro SD cards, and I can tell you it's not a problem with the cards themselves.
Any help is appreciated.