The details are here:

http://www.netbsd.org/Documentation/power-mgmt/

you have to read the:

Power Management Hints and Tricks



Thomas Feddersen wrote:
> Am 30.01.2007, 13:24 Uhr, schrieb Brian McEwen <bmcewen@comcast.net>:
>
>> Does the HD power down with inactivity?
>
> There is no inactivity.
> I can stop the drive from spinning:
>     Qube: {3} atactl wd0 standby
>     Qube: {4} atactl wd0 checkpower
>     Current power status: Standby mode
> but after about one minute it spins up again:
>     Qube: {21} atactl wd0 checkpower
>     Current power status: Active mode
> I suppose, spinning the drive up every minute will put more wear on it 
> than letting it idle. So I didn't set any idle-timer.
>
> If anyone can find a way of having the system not access the drive at 
> one-minute-intervals please let me know.
>
>
> Occasionally I check the status:
>     Qube: {44}  atactl wd0 smart status
> and find everything is reliable after 578 power-on days.
>
> Kind Regards
> Thomas
> Hamburg, GER
>  --
> Dipl.-Ing. Thomas Feddersen
> engineering for pleasure and profit
>