Can't get drives containing spare devices to spindown

["Marc L. de Bruin" <marc@debruin.org>]

Lo,

Situation: /dev/md0, type raid1, containing 2 active devices (/dev/hda1 
and /dev/hdc1) and 2 spare devices (/dev/hde1 and /dev/hdg1).

Those two spare 'partitions' are the only partitions on those disks and 
therefore I'd like to spin down those disks using hdparm for obvious 
reasons (noise, heat). Specifically, 'hdparm -S <value> <device>' sets 
the standby (spindown) timeout for a drive; the value is used by the 
drive to determine how long to wait (with no disk activity) before 
turning off the spindle motor to save power.

However, it turns out that md actually sort-of prevents those spare 
disks to spindown. I can get them off for about 3 to 4 seconds, after 
which they immediately spin up again. Removing the spare devices from 
/dev/md0 (mdadm /dev/md0 --remove /dev/hd[eg]1) actually solves this, 
but I have no intention actually removing those devices.

How can I make sure that I'm actually able to spin down those two spare 
drives?

Thanks,

Marc.