Before I updated my phone to Windows Phone 8.1 dev preview, I had roughly 100 MB of free space. Upon updating, it had suddenly gone down to 0 KB. Yet when I added up the data in the memory stick, it was only close to 1.7 GB, and thus should've had ~100 MB free.
I couldn't figure out what was happening, so I decided to format the memory stick. After formatting, I clearly had all of 1.83 GB free. Then I copied around 800 MB of music and around 600 MB of images. But after that, instead of having the expected ~400 MB of free space, I have just 12 MB. Just like before, the space had vanished magically. (Note that I did nothing other than just copy the files, though I did notice that copying the images took a long time unlike before.)
Even now, if I check the size of all the folders in the SD card, they add up to only 1.35 GB.
Yet, only 12 MB is free.
What is happening here? And no, I haven't installed any apps onto the SD card, and new apps are set to be installed into the phone's internal memory.
I was planning on buying a large SD card now that apps can be installed onto it, but if the space is just going to be eaten up, I'm not so sure.