update , I edited both json files for drive pool and drive scanner and still not working.
Then I found this fix. in my case I was using an LSI raid card flash to HBA mode , I think this is quite common.
I followed option 2 , the regedit version of this solution and the drive pool drives are now sleeping.
now I don't need to listen to 5 drives spinning away doing nothing most of the time and i'm saving 5W x 5 = 25W of power.
My system is low power imbedded Celeron using under 10W but the tv I plug it into is horrendous making the power saving a little bit meaningless, but at least the noise is gone.
and yes , I know permanently running drives can last longer but this is the real world and we have to live alongside these machines