Thanks Jim for the info on sg_utils (which I have downloaded) but they don't seem to offer power management such as spinning down and going into standby after a period of inactivity.
I've discovered this
and it seems to do the trick.
After you've found your usb device in /sys/bus/usb/devices you change the autosuspend and level files as described in that link.
Autosuspending USB devices
To attempt to autosuspend your USB device, first use lsusb as root to find out the bus number and device number of your usb device:
Then find your device's directory in /sys/bus/usb/devices/. Look in directories that are named with two numbers separated with a dash:
Bus 005 Device 014: ID 04b3:4485 IBM Corp.
Bus 005 Device 001: ID 0000:0000
Bus 004 Device 009: ID 0483:2016 SGS Thomson Microelectronics Fingerprint Reader
Bus 004 Device 001: ID 0000:0000
Bus 002 Device 001: ID 0000:0000
Bus 003 Device 001: ID 0000:0000
Bus 001 Device 008: ID 04b3:310c IBM Corp.
Bus 001 Device 007: ID 050d:0121 Belkin Components F5D5050 100Mbps Ethernet
Bus 001 Device 001: ID 0000:0000
We know the USB to ethernet device's directory is 1-2 because the device and bus numbers match the lsusb output. Now we can tell the kernel that it should suspend this device automatically if it is not being used. First we set the idle timeout to 2 seconds:
1-0:1.0 1-1 1-1:1.0 1-2 1-2:1.0 2-0:1.0 3-0:1.0 4-0:1.0 4-2
4-2:1.0 5-0:1.0 5-6 5-6:1.0 usb1 usb2 usb3 usb4 usb5
/sys/bus/usb/devices# cat 1-1/busnum
/sys/bus/usb/devices# cat 1-1/devnum
/sys/bus/usb/devices# cat 1-2/busnum
/sys/bus/usb/devices# cat 1-2/devnum
The timeout can be set to any integer number of seconds. If set to -1, the device will not autosuspend. Then we make sure the kernel will automatically suspend the device, and resume the device if data needs to be transferred:
/sys/bus/usb/devices# echo 2 > 1-2/power/autosuspend
Other options to echo to this file are "on" and "suspend":
/sys/bus/usb/devices# echo auto > 1-2/power/level
* "on" will force the device to be on all the time.
* "suspend" will permanently suspend the device until the user echoes "on" or "auto" to this file.
(Note that this is a simplification, since the value of the power/wakeup file may allow the device to signal a remote wakeup.)
Checkmate, the reason for doing all this is to conserve power and save on wear and tear of the devices.