>> Then maybe the disk driver should keep track of how long the disk has been
>> idle and always send the start/stop unit command if it's been idle more
>> than a specific amount of time.
>
>The driver can't really know how long the disk has been idle because the
>power management is done *inside the disk*.
>And there's no start/stop command for IDE disks.

Well, that's not exactly correct.  There are IDLE and STANDBY commands
that put the drive in the designated mode.  You don't "start" from that;
the next command wakes up the drive.  There's also SLEEP, which requires
a reset to spin up the drive (see atactl(8) for examples).

--Ken