Startup manager is not appearing on mac pro running El Capitan

Dear Experts,

I'd appreciate any help with the following issue:

I have a Mac Pro 4,1 (early 2009) running OS X El Capitan (10.11.6), what I want to do is to access startup manager to be able to lead the system from a different disk for example Time Machine or a bootable flash drive. However, when I turn on the system -> hear the boot sound (startup chime) -> immediately pressing the 'option' (or 'alt') I won't see any thing except a black screen, after which I need to shut down the system in order to access normal booting. I've tried many suggestions e.g. changing monitor resolutions, using wire keyboard/mouse, and many more, nothing works. None of the boot/startup keys (https://www.lifewire.com/mac-os-x-startup-keyboard-shortcuts-2260348) seem to work. Any feedback/suggestions? Thanks.

Best,

FMV