Well,
Just tested
removed windows ssd from optical bay, not any other dd in optical bay
drawer 1 and 2 my MacOS fusion drive
drawer 3 windows ssd
drawer 4 my main volume clone
system pref -> startup disk choose bootcamp
View attachment 760991
reboot
same black screen "no bootable device"
View attachment 760992
I remove bay 3 (win ssd)
reboot -> same black screen "no bootable device"
i put back my game data hdd in bay 3
reboot -> same black screen "no bootable device"
had to reset pram to go back to my main fusion drive
SO:
1) this message display on screen doesn't come from either bootloader (MBR) or Windows as it shows even if disk not in here
2) i still think this is because bootcamp try to find a legacy mbr on the mac pro and not an uefi boot partition as in more recent mac, that is why the emulated legacy bios say that.
BUT
the emulated legacy bios can display something on non mac efi GPU.
if i can make more test, just tell me
*EDIT*
I have to confirm that my windows install is definitely not broken. starting windows with alt at boot (blindly of course) or with win ssd as the only ssd just workd and launch my windows (uefi install)