Dual PSU Issue

My friend just asked me this question a while ago and i cant find an answer to it anywhere so i geuss i will try here.

If I connect a couple of harddrives to an extra PSU, and the PSU is not connected to the moderboard, does it feed them with electricity ? I’m asking this because I need 2 PSUs in my PC so I can turn the second one off when not needed.

Thanks in advance for any help.

Well i just realized there was another thread based on this, but i do not think any of the answers fit what i am needing to find out in this case.