I applaud your caution with usb devices, but I really think you are taking it too far. Unmounting a drive is all that is required. The command you are using is simply powering down the usb port that the device is attached to. How is that different from unplugging it? One second it has the power to spin, next second it doesn't. If it sent a 'spindown' command before removing the power from the usb port it would be different, but I don't read any thing to suggest that it does. Hdparm may be of assistance (although I know from my experience it doesn't work with a lot of Western Digital drives ) but really I wouldn't bother, unmount and unplug.
The best way to protect usb devices from harm (flash drives mostly not usb disc drives) is to disable automounting. If you have to mount something manually you will probably remember to unmount it, otherwise you might not, that is where the danger is, from my point of view automounting should be expunged form the linux kernel forever.
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