I have an old Mac Pro 3,1 running on El Capitan and I was trying to boot from a USB on startup only to see that nothing happens. I can go into Recovery Mode and Safe Boot Mode and my computer runs normally too but the one thing that I can't do is I can't go into startup manager, I've tried to hold the alt key before and after the power on and nothing happens.
Note: I've upgraded my stock graphics card to a newer one so when I boot I don't see the apple logo because the graphics card takes a while to start up, normally, I turn on my mac and hear the chime and wait for it to turn on but my monitor doesn't run, I just have to wait until the initialization has finished for the graphics card to finally start running and my monitor working. When I turn on my mac and hold down alt to get to the startup manager the computer starts up normally but my screen won't turn on and I can't hear my graphics cards fan running which is usually an indication that the start up is almost done so my computer says it's on, my graphics card isn't doing anything and holding alt is useless even for the longest times, I have a feeling it's at the startup manager but my graphics card is still asleep, I can boot normally after a restart.
If there's a solution to this or an alternative method to help me boot from my USB, Its been quite frustrating.