If that doesn't work, the sdparm utility at http://sg.torque.net/sg/sdparm.html
looks like it supports spinning the disk down:
The '--command=' option allows a command to be sent to the given device. The currently supported commands are:
* capacity: sends a READ CAPACITY and if successful reports the number of blocks, block length and capacity expressed in MibiBytes (1048576 bytes). Valid for disks and CD/DVD drives with the appropriate media loaded.
* eject: stops the medium (if it is spinning) and ejects it from the drive. Note that this may be prevented by software in which case use the 'unlock' command first.
* load: loads the medium and then spins it up
* ready: reports whether the medium is ready for IO. Ready usually means that it is present and spun up. If the device is not ready then the exit status will be 2 (see exit status section below).
* sense: reports sense data (from a REQUEST SENSE SCSI command); can include power condition information, a progress indication for a time consuming command (e.g. format) or a report an informational exception (when MRIE=6)
* start: spin up the medium
* stop: spin down the medium
* sync: send a SYNCHRONIZE CACHE SCSI command to the device.
* unlock: instructs the device to allow medium removal (i.e. an eject). Beware, the OS may have had a good reason for preventing removal of the medium (e.g. it contains a mounted file system). Use at your own risk.