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.

Solution 1:

I am adding this as an 'answer' to provide more context as to what solved the OP's issue (and to save others from reading through the comments history). Hopefully this will help someone else in a similar situation.

In the OP's case he has a late 2009 iMac which previously had Snow Leopard (10.6.x) installed. He updated to El Capitan (10.11.x) and found this was incompatible with his music hardware and software. The OP then needed to go back to Snow Leopard and despite having both a Snow Leopard installation disc and a bootable USB installer, he was unable to install Snow Leopard.

The key bit of information that was missing in the original question was that the OP's iMac was one of the quad core models. These were options for the 27" model only and both variants (i.e. the i5 and i7 versions) had a firmware update. Because of the OP's music requirements I had no doubt he would have installed this update (and in fact Apple recommended that all users install it).

However, this prevented the OP from installing Snow Leopard because the firmware update itself requires as a minimum Snow Leopard (10.6.3). The installers the OP was trying to use were obviously for 10.6.2 or earlier.

Summary

If you have a quad core i5 or i7 (late 2009) iMac and can't reinstall Snow Leopard, ensure that the version of Snow Leopard is either 10.6.3 or above.

Solution 2:

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!