You can read about it in the comments:
https://lore.kernel.org/patchwork/patch/856323/(I couldn't find any actual documentation).
For me on GoFlexHome I did this to make orange LED indicate network activity on eth0.
$this->bbcode_second_pass_code('', 'cd /sys/class/leds/status\:orange\:fault/
echo "netdev" > trigger
echo 1 > rx
echo 1 > tx
echo "eth0" > device_name')
And so
tmpfiles does this:
/usr/lib/tmpfiles.d/leds.conf
$this->bbcode_second_pass_code('', 'w /sys/class/leds/status\:orange\:fault/trigger - - - - netdev
w /sys/class/leds/status\:orange\:fault/device_name - - - - eth0
w /sys/class/leds/status\:orange\:fault/rx - - - - 1
w /sys/class/leds/status\:orange\:fault/tx - - - - 1')
Edit: sorry, I misread your post on my phone, I though you were looking for network activity LED. AFAIK there isn't a straightforward one for USB hdd's, so your script should do. The available triggers will only work with hdds attached to sata ports (which E02 has none).