How to switch off your usb external drive after umount and stop its spinning heads?
The following article gives some hints, which I summarize here.
http://www.lesswatts.org/projects/devices-power-management/usb.php
1) find the device to switch off (eg. the USB Mouse)
root@rpolli:/sys/bus/usb/devices# lsusb
Bus 004 Device 005: ID 046d:c050 Logitech, Inc. RX 250 Optical Mouse
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
2) Go to directory
# cd /sys/bus/usb/devices/4-2
3) check that the device is the one we need
# cat product
USB-PS/2 Optical Mouse
4) disable it :D
# echo suspend > power/level
HTH