How do I stop a 2012 iMac internal Fusion drive from powering up
I want to completely stop my internal fusion drive from powering up, since I no longer use it and it is very noisy.


You cannot prevent your mechanical drive from powering up.

It’s a standard 3.5” SATA hard drive with a power connector directly connected to the power supply.  There’s no switch or command you can issue that will cut off the power to the drive.  Repartitioning it and wiping it clean won’t prevent it from spinning up and doing its periodic reads.  



The only solution is to disconnect it. 

However, since you have to go through the trouble of cutting through the adhesive holding the glass, you might as well upgrade the drive to an SSD.  I’ve done this exact procedure - it’s not difficult, just time consuming.