how to stop "Cannot unmount volume" from appearing?

i'm using Hardy Beta. Every time I try to unmount any usb drive, it will show "Cannot unmount volume xxx".
And the "Writing data to xxx" will keep on displaying until I close it.
I didn't see any problem if I unmount from terminal, so I think the error message is just a false positive.
I have this problem since Gutsy, but strangely, it doesn't happen on my other gutsy desktop.