Manually putting external USB drives into standby

You can manually put a USB HDD that doesn’t automatically spin-down after inactivity to sleep with the following command:

sg_start 0 --pc=3 /dev/sdc

where I assumed your HDD was at /dev/sdc.

You can determine which /dev/sd* you’re at with the command

df

The HDD should automatically spring back to active spinning mode when you write a file to it, etc.

You can install the sg_start program via:

sudo apt install sg3-utils

The reason you might want to do this include that the HDDs put out some substantial heat when they’re not spun down. You don’t want to spin up/down too frequently (like every 5 minutes) to avoid excessive wear. If you’re not using the drive for the night or a few days, it might be worthwhile to spin them down if heating is an issue.