I've got an external Seagate HDD hocked up to 18.04 LTS. Unfortunately, even after hours of inactivity the disk will not spin down.
Is there a setting in Ubuntu I haven't found to set an inactivity timer to automatically power down the USB drive when not in use?
Look at this:
https://askubuntu.com/a/884122/831708
Maybe hd-idle works for your System too as described in the link. But I have a similar behavior after upgrading to 18.04 ... on 16.04 all usb drives were spinning down automatically, with 18.04 they are just running :/