It seems like I have at least two options to spin down my external USB drive used for rsnapshot backups (Iomega 1TB). In the first place, I assumed it would spin down by itself by simply unmounting the volume, like on the Mac. But it doesn’t.
So I gave sdparm a try :
sdparm --command=stop /dev/backupdrive
It doesn’t work 
I found a working solution at http://forums.debian.net/viewtopic.php?f=7&t=60122
sg_start --readonly --stop /dev/backupdrive
sg_start is part of sg3-utils package.
YMMV, I guess ?
Use the cmd_postexec option in rsnapshot to trigger the spin down.
Want the same device name for your external USB drive ? Check out http://blog.wains.be/2010/04/10/udev-always-the-same-device-name-for-your-usb-drives/