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