How to downgrade from El Capitan to Snow Leopard? I am not happy with El Capitan, and it is also not compatible with both music hardware and software that I have, so I want to go back to Snow Leopard on my late 2009 iMac.
After installing El Capitan the CD drive on my iMac is not working, and I was unable to install Snow Leopard from it.

So I made a bootable USB that gets recognized when I hold option when the machine is booting. When I choose it, I get the white screen with the Apple logo, but it doesn't go into the installation phase. I have formatted my drive and no longer have El Capitan on it.

How can I install Snow Leopard? It looks to me(and I'm hopefully wrong about this) like El Capitan has blocked the option to downgrade to an earlier OS, and it is a bit infuriating that I am not in control of hardware that I have spent a lot of money on.
As Monomeeth pointed out, a firmware update which requires 10.6.3 might have prevented me from installing from the CD and bootable USB.
I was able to get an image of 10.6.8 and restore it to my hard drive, and I am now finally running Snow Leopard!