strange:

Nokia-N900:/home/user# du -hs
du: ./MyDocs/.maps: Input/output error
395.4M .

.maps is where marble stores the maps

Edit:
ls- la does not show the .maps folder

Hmm marble did corruped some folders on my previous N900. Solution was to rename the .maps folder. I was never able to delete anything because the filenames were strange with all kind of characters.
Maybe marble did something similar again but this time flooding myDocs till it broke.