k0ste, thanks for your suggestions.
I tried option 1 couple of times, but the timing window is so narrow,almost impossible to remove the script before system shutdown.
For option 2,3, looks like DSM system files resides on special partition. I have all 3 hard disks in basic mode. Can I operate files on that partitions?
For option 4, since DSM is in installed status, I don't have the chance to reapply image without formatting hard disk first. Right?