Mac Pro 2010 - Won't boot into Start up manager - Mojave

for mid-2010 to mid-2012 if upgraded with a recommended Metal-capable GPU

macOS Mojave will run on the following Macintosh computers,

• MacBook (Retina)
• MacBook Air: mid-2012 or newer
• MacBook Pro: mid-2012 or newer
• Mac Mini: late 2012 or newer
• iMac: late 2012 or newer
• iMac Pro
• Mac Pro: late 2013 or newer, mid-2010 to mid-2012 if upgraded with a recommended Metal-capable GPU

macOS Mojave drops support for many older Mac models because it relies on Metal, which requires Intel HD and Iris Graphics 4000 series or newer, AMD GCN-based GPUs, or Nvidia Kepler-based GPUs or newer.

You don't list what video card you have and I'm a lousy guesser.

Unless it is one of the original "Metal compatible" Mac cards that shipped with or was sold as Mac compatible then the cards listed as supported from Apple have limited support and will not give you a Loading Screen (Loading bar with grey screen when you first start up) or a boot manager (pressing Option). Not in High Sierra either. If you search the web there are hacks and workarounds for getting them to work. There is a web site that does sell "flashed" metal compatible cards that may give you what you need. Apple may or may not bring full support in the future.

You should be able to use the "Startup Disk" manager in your Preference folder to select any boot-able disks you have mounted in the mean time.

Unless the card is specifically made for Mac or has been 'flashed' to be a Mac video card then you will not get the startup manager screen, or any boot screens.

Can you clarify if it is a Mac edition card, or was flashed for Mac.

I guess I can qualify my statement a little, the made for Apple or shipped with the computer METAL capable cards can be used to uprade the MacPro 2010-2012 to 10.14. without having to jump through hoops to update the MacPro Firmware that's required to run Majave. It seems you have to have an EFI capable card to do both.

It's been a while since I've looked around but here's one I kinda remember.

https://forums.macrumors.com/forums/mac-pro.1/

Have a look through here, this is one of the places I got a head ache from. Some are older 3,1 and 4,1 upgraded to a 5,1 but they explain more then I can.

For example, a SAPPHIRE Radeon PULSE RX 580 8GB GDDR5 is the one that Apple specifically mentions, but it seems some get a boot screen and some don't. Any other version like my MSI Armour RX 580 doesn't get either. Boot or Load screen. (technically it isn't the exact one, it has to be the Pulse) but is one of the Apple "Might be supported" cards. Everything works other wise.

But macvidcards dot com sells the others with a flashed firmware for the MacOS.

Nvida does make a web driver that supports most of their cards but you would have to wait for new drivers every time Apple updates the MacOS at this point.

Sorry I couldn't remember more.

I'm still on H Sierra but when I get time I am going for Mojave.