Thanks Dave!
As you pointed out, i tested my floppy on my friend's computer.  One of
the two "bad" floppies is okay and the other is not.

I guess:
1. i just got a bad (but not too bad) floppy in one situation
2. for the other situation, my new floppy drive may not be good as it
should be (or may not be good as my old one).

As long as my new floppy drive seems okay, i just can't take it to
computer shop (i tried, but it works good there) and exchange one, but
floppy drive is so cheap, i can replace it anytime.

Thanks again!