If I've understood you correctly, I think you'd need to link 0VDC / GND between both the system PSU and the HDD PSU, otherwise you'll get variable reference voltages for the data lines
Happy for someone else to shoot this idea down in flames, but I think the data is using absolute, not differential voltages
Thanks. No need for the setup notes (but thanks for the kind offer), it was more about the experience, but I think you've already answered my question with less surface area (I do have 1 Pi that's internet facing for Radicale)
Have you looked at Ansible? That might also cover what you're trying to do.