Here's the breakdown:

/ is the root system partition where applications are loaded. This is a two Gigabyte partition.

/home/user/MyDocs is the area that is visable from the N900's GUI filemanager as "N900". This is the remainder of the internal 32GB chunk (about 27~ GB).

/media/mmc1 is the Micro SCHC card.

When you plug in the N900 in USB Mass storage mode, the host PC maps the /media/mmc1 and the /home/user/MyDocs partitions as separate readable drives.

If you want to move files from anywhere else onto your PC, first copy them to either the SDHC card, or to /home/user/MyDocs.

Hope this helps.