MP 1,1-5,1 Can’t boot into Mac Pro 4,1 flashed to 5,1 on Mojave after booting to El Capitan.

[Xbfryfd]

So I have a 2009 4,1 Mac Pro that I flashed to 5,1 to upgrade to a Xeon x5680 6 core cpu. I have an rx 480 GPU, 8GB RAM and a 275GB SSD on a PCIe adapter card. I have it on Mojave. I found a good deal on a 2010 dual socket 12 core model that doesn’t come with a hard drive to pick up tomorrow, so I grabbed an old hard drive that had El Capitan on it so I can test it when I pick it up. I put it in the computer and booted into it using the startup disk option in system prefs, just to make sure it worked. Well I had to reinstall the GPU that came with it to see the boot screen. I then wanted to return to using Mojave, but the SSD with Mojave on it didn’t show up as an option in the startup disks. I then changed it to my other SSD with Sierra on it that I was using prior to Mojave, but it wouldn’t boot. I then tried going into the boot drives using option during start up and it took me to a gray screen where the bootable disks should be but there aren’t any disks to choose. I can’t figure out how to boot back into my Mojave disk. Any help would be much appreciated!

edit: I remember there being some weird GPU stuff when upgrading to Mojave so I’m assuming it has something to do with that but I’m not sure. I’m running an xfx rx 480 unflashed.

 

[tsialex]

Remove all other disks except the one you want to boot, do the triple clear NVRAM procedure (press and keep pressed the CMD-Option-P-R until you hear the reset chime 3 times) then try to boot from the Mojave drive.

Keep just one GPU installed. You can't mix match NVIDIA and AMD GPUs with Mojave.

 

[Macschrauber]

Maybe the 5.1 is on older Firmware with lack of NVMe or even APFS Support.