There is a Windows 10 EFI/GPT installation on a separate drive in my MacPro next to my disk with the OSX installation.
The Apple Startup Manager shows the Win10 disk labeled as "EFI Boot". I can boot to Win10 there.
I cannot set the default startup disk in system preferences. If I choose the Win10 HDD I will get "No Bootable Device -- Insert boot disk and press any key" error upon next boot.
As soon as I bless the OSX disk again through system preferences OSX will boot again as standard.
How can I "bless" the Win10 disk as the standard disk to boot upon startup?
There is no EFI partition present on the Win10 disk.
diskutil list output:
/dev/disk6 (external, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *250.0 GB disk6 1: Microsoft Reserved 16.8 MB disk6s1 2: Microsoft Basic Data BOOTCAMP 249.5 GB disk6s2 3: Windows Recovery 471.9 MB disk6s3 /dev/disk7 (external, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *256.0 GB disk7 1: EFI EFI 209.7 MB disk7s1 2: Apple_HFS Macintosh HD 255.1 GB disk7s2 3: Apple_Boot Recovery HD 650.0 MB disk7s3
UPDATE: Windows installer seems to have used the EFI partition on disk7 to copy the boot files to. I managed to "repair" the startmanager with this guide here. Windows now boots on every startup of the MacPro. I can also access the Apple Startup Manager and choose my Macintosh HD or the Recovery Partition. As soon as I mess around with choosing the startup disk in osx system preferences it will break again with the "no bootable device error". OSX or the Startup Manager does not seem to be able to find the Windows OS on the other disk6 without EFI Partition. Interesting also is that even when I disconnect the windows disk7 I will still see EFI Boot in Apple Startup Manager. Choosing it will get a Windows Error screen not being able to boot of course.
As soon as I have more time I will continue to experiment further... Maybe somebody with a better understanding of the Apple EFI Startup Manager can give a hint in advance how to make it possible to switch startup disk from OS side in a "normal" way.
Next step to try: Add EFI partition to Windows disk6 put the Win startup files there and link to the installation on the same disk.