Hard Disks do NOT sleep (now - they used to!)

In Energy Saver preference, I have checked "put hard disks to sleep when possible." For a long time, selecting SLEEP from the Apple menu resulted in the hard drives (external, thunderbolt) going silent after a minute or two.

Now the hard drives NEVER turn off, I continually hear them making the sound as if the system was turning on and booting up, forever!! Just noticed this after returning from a week's vacation (computer off).

What could be causing this?? This machine is primarily used for video editing. It talks to dropbox, but that has been true for several years. Still running Mojave - did the last security update ruin this?