External USB HDD is not spinning down

We automatically run hdparm on external drives when they are automounted:

However not all external USB drives support this, so it’s highly likely that your new drive doesn’t support the spindown commands sent by hdparm. I have two external 2.5" drives and neither one of them supports spinning down with hdparm.

Try running the following command and see whether it reports an error:

sudo /sbin/hdparm -S 240 /dev/sda