MP 1,1-5,1 startup disk set to Bootcamp prevents to boot from Mac OS (no boot screen)

[Bruudinho21]

I purchased a Mac Pro 5.1 mid 2012 but wasn't aware that the installed XFX RX580 doesn't provide a boot screen. From my former Mac Pro I have a bootcamp drive which I put into the new Mac Pro. But I couldn't boot into Windows for undefined reasons. So I converted the Win partition from MBR to GPT and was able to set Bootcamp as startup disk. After restart I see a black screen saying "no boot device found" which means the boot sequence is not compatible.
Because I don't have a boot screen when holding ALT I can't select Mac OS as boot device. Is there any chance to change startup disc without exchanging the graphics?
I even can't start into recovery mode.

Thanks for helping me!

 

[startergo]

NVRAM reset
[automerge]1590851744[/automerge]
Do you have OSX installed on another drive? If not it will always go to Windows. Insert a USB or DVD installer for OSX and use option button at boot.

 

[Bruudinho21]

Off course I have OS X Mojave installed on my system drive and Win 10 on a 2nd drive. But I can't use option button at boot because my RX 580 graphics doesn't show a boot screen.

 

[startergo]

Off course I have OS X Mojave installed on my system drive and Win 10 on a 2nd drive.

Then NVRAM reset should work.