The moment you plug your N900 into your computer USB port, it will unmount the vfat partition (because it gets mounted as a flash drive on your PC instead).
Once you properly eject it from Mass Storage mode, it will get mounted again.

I had the same problem with my phone. It lost all memory except 455 mb, and would not run any programs due to lack of memory. I turned it in for service, but they could not fix it and has shipped it to Nokia. I'll have to wait 2-3 weeks for an answer. Sucks!

same problem is here
i lost all 32GB intenal memory except 445.5MB
madness !!!!!!

As posted in the other thread. I've seen a couple of instances of this in the Nokia forums. But in their case it was caused by dropping the device or giving it a sharp accidental knock.
Turned out they had dislodged the EMMC module from the mainboard.

thanx for replying but i have one more question .....
dislodged or damaged ????
is there a chance of being repaired in a service center???

Either one, dislodged or damaged, it would still need to be repaired.

I'm not too savvy with the actual internals of the N900, so not exactly sure how the EMMC module is installed. Not sure if it's surface mounted (soldered directly to the board) or a plug in type module (like the BIOS in a PC).

Sorry for double post.

Just found one of the relevant threads on the Nokia forum:

http://discussions.europe.nokia.com/...ng/td-p/678373