who can turn off harddisk

I'd like to turn off my harddisk for saving power ,so the option for
harddisk has been change to "turn off after 15 mins".
But almost every one hour,the second harddisk will be waken up(the primary
harddisk with os,the second harddisk just for storage,first one is of
sata,second one is of pata).Obviously,I'm going to make the second one have a
good rest.

Even if I don't run any program except services, it still happens regularly.
so I was wondering which service will wake it up,shall I need to amend some
key in registry.

By the way,I installed norton antivirus and thid party firewall,meanwhile
there is no any scheduled scan or actions.
Many thanks