Do spare drives sit idle?

I have a RAID 1 setup on my machine. I have a 3rd drive not attached as a hot spare but it is there in case I need it. My question is if the spare drive is not attached to an array and not being used, will it sit idle or will it be powered on and spinning even though it is not being used? If so, can it be disabled without removing it or would I be better off using it as a hot spare? Thanks!

I recommend to use it as a hot spare and have it ready but if you are concerned with power consumption then you can leave of then power on to use to replace the bad drive.