Hey! I am trying upgrade my old imac (10.5.8) OS to a newer one. Made bootable usb and dvd with Snow Leopard and El Capitan, but none of these bootable things are working. It just does not see any of the bootable things at the startup. Made the external drives with different bootable ways (using Disk Utility and Terminal) on different macs - nothing is showing up during startup. Upgraded all SMC firmware and all other possible upgrades. At the Preferences/startup disk the external volume shows, but it does not make sense, as it is not booting from any external drive by pressing C or Option key during startup. Help please!!!!