There are background programs that keep the hard disk spinning. This one might be useful.I have a new iMac 5K, to replace a Late 2012 iMac whose screen/graphics card was failing. Mojave has been foisted upon me. I have a hard drive that is a dual TB1/USB 3.0 interface drive. If I connect it with the one TB2 to TB3 adapter I own, it stays spinning. If I connect it via USB 3, it will spin down even though I have gone into Energy Saver in System Preferences and unchecked "Put hard disks to sleep when possible." I have tried checking it, closing System Preferences, and removing the check again, but to no avail. Is there anything I can do, or is this feature fundamentally broken over USB 3?
I also know it's not specific to this system. The drive exhibited the same behavior while attached to my mom's High Sierra iMac while awaiting the new iMac.