I've try this few days:
Unmount the drive --> the LED goes to standby mode (the drive seems stop running, no noise) --> then press the power button --> the LED turn off
I keep FW 800 cable connected and the computer can still be on.
But when I turn on the computer next time, the drive will keep in off status (LED off, but strange enough, the drive has no noise and not running but it feels a little bit warm, don't know why), then I can press the power button and the drive will mount by itself.
If this is the right way to turn off the drive, then it seems acceptable.