Gramps, to try and explain without too much BS. As you know the hard drive is a spinning mechanism. If you don't need it spinning then there is no need for it to be spinning. There is wear and tear involved with spinning, as there is in getting it up to speed. What is that point of trade off? NO ONE can tell you that.
I would put a rough guess, that if you have not used the drive in the past 10 minutes, then it might be reasonable to turn off the power to it. However, unless you are talking about laptop computers there is very little advantage in turning off the hard drive during even longer periods of times.
So, for desktop computers I would usually not even worry about turning off the hard drive. The bearing mechanism in today's hard drives is sealed to a point where the wear in not even worth worrying about anymore.
For desktops, if I were to used some time to turn off the HD, I would put a shut-off time in the order of an hour or more. I figured that if I haven't accessed the hard drive in the past hour, I probably have gone to bed.
What happens, what are the disadvantages, of having the hard drive de-powered, and you need something off the hard drive? Simply that it will take an extra second or two to get to the data. Big deal. I am not in that much of a hurry, that an extra second is going to make me miss a schedule. Besides, with a HD turn-off time in the 10 to 60 minute range, it is most likely that the hard drive was accessed in less time than that, if I was actually using the computer, anyway.
About turning off the hard drive in hibernation, I don't think you have a choice anyway. I think it ALWAYS de-powers when hibernation is performed. It makes no sense to have the hard drive going otherwise. Hibernation is where the computer copies the entire real RAM memory image out as a single file to the hard drive. All for the express purpose of being able to remove ALL POWER from the computer.
Hibernation is different from Stand-by. Stand-by is where a small amount of memory power and cycling is maintained to keep the data in RAM. Even then, even in Stand-by, the spinning of the hard drive is not necessary. The hard drive is not needed to be accessed in Stand-by, and it definitely is not needed, for any reason, in Hibernation.