I couldn't seem to find this exact situation when I searched so apologies if this has turned up before... I recently sold a Mac Pro 3,1 running El Capitan. This is an octocore (2 x 3.2GHz Xeon) with 16GB. The guy I sold it to apparently wanted Snow Leopard on it (I assume he has some legacy software he wants to run). When he moved an SSD from one Mac Pro running Snow Leopard to the one he bought from me, he got kernel panic. He tried installing from the Retail DVD and again got kernel panic. He then made a bootable USB with Snow Leopard and tried installing again. And again, kernel panic. When he puts the original SSD I shipped the Mac with, it boots up to El Capitan every time with no issues and everything works (including the superdrive). I've confirmed with him that all peripherals are disconnected other than keyboard. He has tried multiple USB drives. He even says when he puts a new drive in and attempts to install El Capitan from a USB that it just hangs and freezes and never completes the install. He has tried multiple HDDs for the install. Since everything works in the original configuration, I'm inclined to think this might be user error. But I'm wondering if anyone knows of something that might cause this issue. Taking this thing back is going to be a major pain so I'd like to avoid it if possible.