given the map dir, i suspect what your looking at is the internal memory card of the N800.

using the tablet file manager as a reference:

the area named "Nokia N800" can not be accessed via usb. this is part of the file system that the tablet uses to store programs and the os itself (and no, you can not move programs elsewhere), so avoid storing large files there as it will take up space best used for programs.

"Internal memory card" is the card under the battery cover. when the usb cable is plugged in, it will be removed from the file manager and made available to the pc via the cable (unless one have set up cache on the card and its being used).

"External memory card" is the card found under the stand. this will also be removed from the file manager and made available to the pc when usb is plugged in.

the reason for it being this way is partially related to how linux handles storage devices, and partially for ease of development.

when you hook a hardrive or similar to a linux machine, its not given a drive letter like in windows.

instead one take a empty folder/directory and "mount" the drive to it, showing the content of the drive as the content of the folder.

the N800 automates this with the help of sensors in the battery cover and the hinged card cover, "unmounting" the card(s) depending on the condition of said sensors.

the same unmounting happens when the usb cable is connected.