Moving on :-
5. I re-create the second partition as ntfs, and copy the
entire KNOPPIX directory (together with knoppix-data.img ),
and rename the existing KNOPPIX directory to KNOPPIX.bak
( All these performed offline )
6. Then I reboot the system, now /mnt-system is sit on /dev/sda2.
I check knoppix-data.img to see if it is unmounted cleanly,
yet it did.
Moving on :-
7. I increased the size of knoppix-data.img to much bigger ( 2.5G),
can copy a huge /home directory ( about 800 M ) to knoppix-data.img
file system. Again all these performed offline.
8. Now test boot/reboot, knoppix-data.img is never cleanly unmounted
after that ( even though I e2fsck on it each time I repeat test ).
Now I am not sure if a huge knoppix-data.img which is causing the problem,
or there are something in the /home which causes it to be unable to unmount
properly.