A concern I had when I installed an RX580 Card into my cMP 3,1 was not getting boot options. This was solved first by Refind and later by OpenCore. However, I have stumbled on a way to get this natively while tinkering.
If you shut down, pull out the current bootable disk, and reboot while pressing the NVRAM Reset keys (OPT, CMD, P & R) and allow three consecutive chimes, you will be kicked into the native recovery screen.
First screen will be one asking for language. Moving on from this will bring up the options to recover from time machine etc.
While that screen is up, clicking on the Apple Icon in the menubar will give you the option to select a start disk and reboot.
Obviously not something for day to day use but useful to know.
If you shut down, pull out the current bootable disk, and reboot while pressing the NVRAM Reset keys (OPT, CMD, P & R) and allow three consecutive chimes, you will be kicked into the native recovery screen.
First screen will be one asking for language. Moving on from this will bring up the options to recover from time machine etc.
While that screen is up, clicking on the Apple Icon in the menubar will give you the option to select a start disk and reboot.
Obviously not something for day to day use but useful to know.