How can I make a selected internal HDD spin down when idle

My D: drive is only used for backups (mostly) and I want to have it spin down when it has been idle for a while. If I use Power Options that would sort of work as the system disk isn't idle for long, except when the PC is unattended. But that causes one of my external drives to vanish, so it's not a good option.
Back to focus: make D: spin down after 20 minutes idle, leave other disk power settings unchanged. I think there is a registry tweak but I can't find the article about it.
The PC is on 8.1 update 1.

Did you look in Control Panel, Power Options? I haven't checked but there may be a power setting for each HDD versus all HDDs.

I don't think this is possible with 1 drive while not affecting the others. So what you might consider is pulling the drive and installing it into a docking station or external enclosure, then power up only when needed.