If you connect the N900 to a computer via USB frequently, you will sooner or later encounter this error where the N900 will refuse to be used as external media.

This is because a program (or programs) is accessing files in the /home/user/MyDocs folder. To fix the problem, the program(s) in question needs to be found and stopped. To find the program(s), open a root terminal and type

 lsof | grep MyDocs

This will display the programs accessing the MyDocs folder something similar to this:

Nokia-N900-42-11:~# lsof | grep MyDocs
sh        3474       user  cwd    DIR      179,1    65536       4326 /home/user/MyDocs/tmp

The second column displays the process-ID of the program (in RED above). Now, terminate the program with this command (repeat for multiple programs):

kill 3474

Now, run the lsof command again (press UP arrow to go back) and make sure the program is not displayed. If it is still running, execute this:

kill -9 3474

Reconnect the USB cable and you should now be able to use the N900 as external media.

Now we have some interesting news for you all

Here are the pictures showing N900 can be used in portrait mode too, So it seems like they have already implemented and now taking their time to make it work flawless on accelerometer

It just matter of weeks to get the portrait mode working.

Here is the video

http://share.ovi.com/media/Guyver95.PortraitMode/Guyver95.10009?sort=0

Source