I am running Windows Phone 8.1 (8.10.12397.895) on a Nokia Lumia 810.
Recently, I put an 8 GB microSD card in to the phone only to find the following.
So the memory card is not visible to Storage Sense but the phone is actively using the memory card.
I found someone else who seems to have reported the issue here. However, the up-voted workaround doesn't work for me because my phone doesn't stay switched on without the battery in.