I was noticing that the space on my SSD was shrinking precipitously, so I decided to look through the folders. Lo and behold, AppData is taking up 10 GB of space! I've attempted to move it (per What is a safe way to move AppData\Local to a different drive?) but every time I attempt to do so I am stopped with an error message about UsrClass.dat.
So I have basically run out of space on my s4. I went thru the app manager and hit the move to SD card on all apps. Very little space there is used. I suspect pictures and video are taking too much space. Right now I use the gallery app to view organize and upload imaged. Is there an app to replace gallery that would move them all to the SD card and still work easily with them?