The Operating System is not included in that ~35MB of installed files (i.e. the Linux kernel, the GNU components, the desktop, the media player and associated components, etc.)

If you open a terminal and "cd /" (change to the root directory), then run the comand "du" it will tell you how much data is in each directory on the device - note that this will be a very long list so best to pipe it somewhere with lots of space and look at it on a big screen, it will also take a while to run.

E.g. "cd / && du>/media/mmc1/du_output.txt" or similar


About your internal card, sounds like it might have got corrupted. What does the output of "mount" say? (i.e. we want to know if it's mounted rw - read/write or just r - read-only). If the later, you should unmount the card and run fsck on it, but we'll get to that when we see the output of mount.