How do put an ATA disk into sleep/standby mode?

In my workstation, running 5.1-R, I have two disks, one containing
FreeBSD, the other Windows.  Since the Windows disk isn't used at all
when in FreeBSD, I would like to put it into standby or sleep mode
(whichever is necessary) to make it spin down, reducing heat and noise
production--both of which have become a real problem.

This is at least technically possible, since spinning down a disk is
part of the APM sleep mode.  The capabilities list atacontrol reports
for the disk in question shows power management and advanced power
management as being supported, and I've gotten Windows to spin down this
disk.  I haven't had any luck figuring out how to do this in FreeBSD,
though.

So how do I accomplish this?