Hi! I have a cMP 4.1 -> 5.1 with Mavericks 10.9.5 installed. All the original hardware except Intel Xeon W3690. Made a bootable El Capitan install usb, but it does not show up in Startup Disk selection screen. When I try to hold down the Option key while booting, Mac hangs on white screen.. The same USB instalation shows up in Yosemite boot screen on my MacBook Pro. Tried PRAM reset and NVRAM reset, nothing works. Mavericks boot selection screen is just broken! So do I have to update Mavericks to El Capitan and then do a clean install? It just takes twice as much time..