Hmmm.... I guess the version of du on the N900 doesn't like the human readable flag.

Can you try this:

cd /media/mmc1

Then type:

du .

(don't forget the space and period after the letters du)

here

32 ./My_Images
32 ./My_Music
1883680 ./My_Videos
1953504 .

This is showing 1.8 MB of space used in the SD card's Videos folder and 1.9 MB used in the card's top level. This can't come close to filling up your card unless it's only got a primary partition of like 5 MB of formatted FAT32 space.

Type this to so us the formatted usage percentage:

cd /media/mmc1

then type:

df -h .

and paste the results here.

filesystem Size Used Available Use% Mounted on
rootfs 227.6M 148.4M 75.1M 66% /
ubi0:rootfs 227.6M 148.4M 75.1M 66% /
tmpfs 1.0M 104.0k 920.0k 10% /tmp
tmpfs 256.0k 92.0k 164.0k 36% /var/run
none 10.0M 80.0k 9.9M 1% /dev
tmpfs 64.0M 4.0k 64.0M 0% /dev/shm
/dev/mmcblk0p2 2.0G 332.3M 1.5G 17% /home
/home/opt 2.0G 332.3M 1.5G 17% /opt
/opt/pymaemo/usr/lib/python2.5
2.0G 332.3M 1.5G 17% /usr/lib/python2.5
/opt/pymaemo/usr/share/pyshared
2.0G 332.3M 1.5G 17% /usr/share/pyshared
/opt/pymaemo/usr/lib/pyshared
2.0G 332.3M 1.5G 17% /usr/lib/pyshared
/opt/pymaemo/usr/share/python-support
2.0G 332.3M 1.5G 17% /usr/share/python-support
/opt/pymaemo/usr/lib/python-support
2.0G 332.3M 1.5G 17% /usr/lib/python-support
/dev/mmcblk1p1 1.9G 1.9G 0 100% /media/mmc1
/dev/mmcblk0p1 27.0G 6.5G 20.5G 24% /home/user/MyDocs

could your memory card be broken?

So, just so I know that we're reading this correctly: Is this a 2GB MicroSD card?

And you just formatted it FAT32 before you put it into your N900, right?

Can we see a long listing of the contents?

Type:
cd /media/mmc1

Then type:
ls -al

I smell bad sectors here.... try typing this.... as root
"umount /home/user/MyDocs"
"mkfs.vfat -F 32 /dev/mmcblk1 "

Then
"reboot"