I use hdparm -Y to put sdb and sdc to sleep. A scrip monitors /proc/diskstats to determine if the disks are inactive and puts them to sleep if there is no change. Webmin 1.560 accesses the disks frequently changing /proc/diskstats and prevents them from sleeping. Even if I manually put the disks to sleep they get woken up. Reverting to Webmin 1.540 removes this problem. I did not try webmin 1.55. Webmin should not be accessing storage drives when no commands to do so are given.
OS: Ubuntu 11.04 Server
Webmin installed on sda
storage disks: WD15EARS & WD20EARS